Better errors on push

Backport #1023 for v1.1.x: Better errors on push

Our "commands" package includes several utility functions which
log or output error messages before causing the Git LFS client to
exit immediately by calling the Exit() function from the "os" package
in the Go standard library.

The Exit() function in our "commands" package, which was added in
commit 934408e, writes a formatted
message to the standard error file descriptor before causing the
client to exit with a non-zero status code.

The Panic() function also performs these actions, but also expects
an additional "error" type parameter, whose message and associated
stack trace the function writes to a log file in the ".git/lfs/logs"
directory.  The initial version of the Panic() function was first
added in commit 099c2f1.

Both the Exit() and Panic() functions pass a status code of 2 to
the Exit() function of the "os" package.

In addition to these utility functions, in commit
090944e of PR git-lfs#1023 we added the
ExitWithError() function to our "commands" package.  This function
expects a single "error" type parameter, and then passes this
parameter's error message to the Exit() function or passes both
the parameter and its message to the Panic() function based on
whether the error is considered non-fatal or fatal according to
the IsFatalError() function from our "errors" package.

However, the implementations of many Git LFS commands simply call
the Exit() function from the "os" package directly.  In these cases,
the commands want to cease operations and halt but do not have
an error message they want to output.  As well, they often want to
return a status code other than 2, which our Exit() and Panic()
functions always return.

We now replace all these direct calls to the "os" package's Exit()
function with calls to a new ExitWithCode() utility function.  This
new function simply calls the Exit() function from the "os" package
and passes the given status code as its parameter.

By consistently using our utility functions to call the Exit() function
of the "os" package, we minimize the number of changes that will be
required in a future PR to guarantee that our commands always perform
a common set of cleanup steps before exiting.

Note that we also take this opportunity to remove one extra line of
whitespace from each of two source files, specifically those for the
"git lfs post-checkout" and "git lfs post-commit" commands.

Our "commands" package includes several utility functions which
log or output error messages before causing the Git LFS client to
exit immediately by calling the Exit() function from the "os" package
in the Go standard library.

The Exit() function in our "commands" package, which was added in
commit 934408e, writes a formatted
message to the standard error file descriptor before causing the
client to exit with a non-zero status code.

The Panic() function also performs these actions, but also expects
an additional "error" type parameter, whose message and associated
stack trace the function writes to a log file in the ".git/lfs/logs"
directory.  The initial version of the Panic() function was first
added in commit 099c2f1.

Both the Exit() and Panic() functions pass a status code of 2 to
the Exit() function of the "os" package.

In addition to these utility functions, in commit
090944e of PR git-lfs#1023 we added the
ExitWithError() function to our "commands" package.  This function
expects a single "error" type parameter, and then passes this
parameter's error message to the Exit() function or passes both
the parameter and its message to the Panic() function based on
whether the error is considered non-fatal or fatal according to
the IsFatalError() function from our "errors" package.

However, the implementations of many Git LFS commands simply call
the Exit() function from the "os" package directly.  In these cases,
the commands want to cease operations and halt but do not have
an error message they want to output.  As well, they often want to
return a status code other than 2, which our Exit() and Panic()
functions always return.

We now replace all these direct calls to the "os" package's Exit()
function with calls to a new ExitWithCode() utility function.  This
new function simply calls the Exit() function from the "os" package
and passes the given status code as its parameter.

By consistently using our utility functions to call the Exit() function
of the "os" package, we minimize the number of changes that will be
required in a future PR to guarantee that our commands always perform
a common set of cleanup steps before exiting.

Note that we also take this opportunity to remove one extra line of
whitespace from each of two source files, specifically those for the
"git lfs post-checkout" and "git lfs post-commit" commands.

Our "commands" package includes several utility functions which
log or output error messages before causing the Git LFS client to
exit immediately by calling the Exit() function from the "os" package
in the Go standard library.

The Exit() function in our "commands" package, which was added in
commit 934408e, writes a formatted
message to the standard error file descriptor before causing the
client to exit with a non-zero status code.

The Panic() function also performs these actions, but also expects
an additional "error" type parameter, whose message and associated
stack trace the function writes to a log file in the ".git/lfs/logs"
directory.  The initial version of the Panic() function was first
added in commit 099c2f1.

Both the Exit() and Panic() functions pass a status code of 2 to
the Exit() function of the "os" package.

In addition to these utility functions, in commit
090944e of PR git-lfs#1023 we added the
ExitWithError() function to our "commands" package.  This function
expects a single "error" type parameter, and then passes this
parameter's error message to the Exit() function or passes both
the parameter and its message to the Panic() function based on
whether the error is considered non-fatal or fatal according to
the IsFatalError() function from our "errors" package.

However, the implementations of many Git LFS commands simply call
the Exit() function from the "os" package directly.  In these cases,
the commands want to cease operations and halt but do not have
an error message they want to output.  As well, they often want to
return a status code other than 2, which our Exit() and Panic()
functions always return.

We now replace all these direct calls to the "os" package's Exit()
function with calls to a new ExitWithCode() utility function.  This
new function simply calls the Exit() function from the "os" package
and passes the given status code as its parameter.

By consistently using our utility functions to call the Exit() function
of the "os" package, we minimize the number of changes that will be
required in a future PR to guarantee that our commands always perform
a common set of cleanup steps before exiting.

Note that we also take this opportunity to remove one extra line of
whitespace from each of two source files, specifically those for the
"git lfs post-checkout" and "git lfs post-commit" commands.