Adds emitting of errors on spawn or Sass error

This adds proper emitting of errors to the plugin. Sass and Bundler log some "errors" to stdout, so we catch those with regular expressions and emit them as errors. Errors will be emitted when:

- Sass has compiling errors
- Bundler isn't installed
- Gemfile is missing
- No Gemfile version of Sass is available
- Sass is not installed
- Any normal spawn error

Closes #94