Gracefully handle errors in linters

Previously, if a linter raised an error `scss-lint` would crash with no
helpful information about which SCSS file caused the error.

Change this to report a nice error message with the name of the linter,
the file it failed on, the original error message, and the original
backtrace for the error. Also include a message with the URL for the
GitHub repo so the developer can easily report bugs.

Also included for free is some class-level documentation and a few
Rubocop lint fixes.

Closes #44

Change-Id: I892b6cc2bfd365b84fdd83b47c467679e0a5b781
Reviewed-on: https://gerrit.causes.com/30682
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>