Add option to raise exception on internal errors

[TV4Fun]

For using external debugging tools like PyCharm, the most helpful thing
to do on an internal error is to just raise an exception, rather than
printing a stacktrace or dropping to PDB.

[gvanrossum]

I guess it's arguable what's the right thing to do though. What does your caller want to do with the exception, other than print a traceback?

OTOH maybe the code that calls report_internal_error() should check this flag instead, and just call raise (to re-raise the original exception) so it can be caught and debugged in its full glory. I'd use that occasionally when developing mypy.

[TV4Fun]

@gvanrossum report_internal_error() is called in 5 different places in Mypy from different places that can catch exceptions, and it seems like it would be bad coding practice to require all of them to include exactly the same check, when we already have report_internal_error() specifically to handle internal exceptions in accordance with the user's preference. Re-raising the original exception as this does preserves the stacktrace of the context that originally produced the exception, which is generally what we want for debugging. My particular use case in this is using PyCharm to debug changes to Mypy. Pycharm will break on exceptions and show you the exact location in the code as well as the values of all variables in every scope, which is quite helpful. Printing a stacktrace does not usually give enough useful information by itself to check an error, and while I'm sure I could probably get this same information from pdb, PyCharm provides a more visual interface, and I am more comfortable with it.

[gvanrossum]

OK, you've convinced me. That use case seems legit.