Fix exception-related PEP8 issues

This changeset resolves 3 PEP8 issues: It addresses the following
exception-related PEP8 violation:
- W602 deprecated form of raising exception
I'm unfamiliar with the extra arguments after the error itself - what do they do? (just want to fully understand before merging, as Capa is a notoriously untested, f'ed up area of the code base)