feat(reporter): nicer output for 'fail'-reporter

Throw a PluginError instead of a standard one in fail reporter,
so that the fail output is a focused couple of lines nicely formatted
by gulp instead of a big noisy stack.
Merge pull request #20 from ejoubaud/plugin-error-for-nicer-output

Ammended commit with `feat` instead of `fix`
feat(fail-reporter-api): Stop failing on errors

- Consider a run successful only if there are no errors, regardless of
  warnings
- Have the fail reporter break the build only if there are errors,
  regardless of warnings
- Have the default reporter show up on both warnings and errors, not
  just on failure
- Add a new `failOnWarning` reporter to fail on both warnings and
  errors, reproducing the previous behaviour of the `fail` reporter

BREAKING CHANGES:
- Set `file.success` to `true` and fail `fail` reporter only
  when `file.errorCount` is 0, even if `files.warningCount > 0`
- Show `defaultReporter` whenever `file.warningCount > 0` or
  `file.errorCount > 0`, regardless of `file.success`