This PR does 3 different things:

1. Adds functions and macros to be used with Status and StatusOr<T> constructs, so as to make it easier to propagate non-ok status.
2. Introduces XLA_SHOW_CPP_ERROR_CONTEXT environment variable for toggling error context.
3. Refactors XlaCoordinator to use the new status propagation constructs as an example

Mainly, inspired by tensorflow implementation, it introduces the following status propagation functions:

• XLA_ASSIGN_OR_RETURN(LHS, REXPR, ...) , which either assigns the value held by REXPR to LHS if it holds a non-ok status code, or return the non-ok status.
• XLA_RETURN_IF_ERROR(EXPR, ...), which early-returns if EXPR is a non-ok status, propagating the error to the caller.
• XLA_ERROR_WITH_LOCATION(STATUS), which builds a new absl::Status from STATUS, by maybe appending the current file location

C++ Error Handling

Idea: by default, print only the user targeted message, with no extra C++ details. Then, by setting the XLA_SHOW_CPP_ERROR_CONTEXT environment variable, also print extra context information, such as C++ source location, and other contextual messages.

• Callers may overwrite the error message by specifying the last optional argument of XLA_ASSIGN_OR_RETURN and XLA_RETURN_IF_ERROR. Rationale: they might have more context to create a more user-friendly message.
• If XLA_SHOW_CPP_ERROR_CONTEXT is set, the callee overwritten error messages are shown in the following lines

Example errors:

• When XLA_SHOW_CPP_ERROR_CONTEXT=0 (default):

RuntimeError: <message>

• When XLA_SHOW_CPP_ERROR_CONTEXT=1:

RuntimeError: <message> (at <file>:<line>)
From Error: <previous-message> (at <file>:<line>)
...