Print, but don't fail on deprecation notice

A simple change so that deprecation notices are printed, but don't cause test failures. A possible fix for issue #27.
I think it'd be worth extracting the second return argument to its own variable, `has_errors`. Otherwise the logic in the `return` line is a little difficult to read.

I like the way you've renamed `errors` to `output` to make it clearer that it may contain either errors or warnings.