Greatly improved error recovery & reporting in Kitchen::Loader::YAML.

Now on error when parsing YAML config files or processing ERB, a more
helpful message is displayed to the user, instructing them to run
`kitchen diagnose --no-instances --loader` for more details.

More care was taken to handle specific file loading errors, and a
special case when an entire YAML file has been commented out with hash
characters at the beggining of lines.

Additionally, full spec coverage is in place and ready for future bug
and regression cases.

Closes #285