Show file name above formatted warnings and errors

[sezna]

Now that multiple files can be compiled at once, we need to include the filename in errors and warnings. This will touch a lot of lines of code, so I think it should be a separate PR from the implementation of include statements themselves.

[emilyaherbert]

This is completed, right?

[sezna]

We can close this one if you'd like, but it isn't really fully completed. See #276