Improve behavior for detecting the failure to parse external linter output

Summary:
Currently, a lot of ArcanistExternalLinter subclasses have something like this:

if ($err && !$messages) {
  return false;
}

We can avoid this code duplication by moving this check to the ArcanistExternalLinter class.

Test Plan: arc unit

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11321