This is work started by @mvdbeek with PR galaxyproject#281.

Refinements include:

 - Using an abstract object instead of simple string for indicating tool failure.
 - Extending shed_lint --tools with the failing behavior.
 - Not having other, non-lint uses the tool directory loading register tool errors.*
 - Adds test cases for both shed_lint and lint.

* It may in fact be the case that these should handle this error, but they weren't really checking for the string "error" so I was worried these would just fail with obtuse error messages. This should be revisited down the road I thin.