separate out errors into their own file, and make them slightly more helpful, fixes #9
adverbs, man
make 'did you mean' for relative URL error message work
CheckFailed.explain_error -> .message
move lib/errors.rb to lib/jekyll_sanity_checker/errors.rb so that it doesn't clobber other errors.rb
Merge pull request #14 from fastly/edmund-clearer-more-helpful-errors

separate out errors into their own file, and make them slightly more helpful