Improvement: Handle non-UTF8 files

[DaveMcEwan]

Address #78, in prep for comments on https://github.com/dalance/svlint/pull/224.

• Add tests for Error::DefineNoArgs, Error::DefineNotFound, and Error::DefineArgNotFound.
• Extend Error::DefineNoArgs with single argument to show which macro requires arguments.
• Add Error::ReadUtf8, with a test, for files of invalid UTF-8. Tested on sibling branch of svlint.

[dalance]

Thanks! I released v0.12.3.