Update MySQL Replication's alerts to include replication errors

I was debugging some alerts I saw where Scout reported the replication as stopping, but started again within minutes. I hadn't had any luck finding any cause, so figured it would be useful to know what errors, if any, replication was reporting.

Also, I was going to update the tests, but they all seem to be failing on master for this plugin. I can look at fixing those, but will do it as a different branch/pull request.