Feature request: option to treat warnings as errors

[jfly]

Right now, nil diagnostics returns 0 (success) if the highest level diagnostic was a warning (relevant code).

I'd like to use nil in CI to enforce that there are no warnings in my code, so I'd like nil diagnostics to return non-zero if the highest level diagonstic is an error (basically, "treat warnings as errors", AKA the classic --Werror compiler flag).

I could try to put together a PR for this if you're OK with the idea.

Thanks for the great tool!