Add more specific class to cite errors

Problem:

* VisualEditor needs to be able to more reliably target Cite errors.

Solution:

* Add a more specific class to cite errors

Grumble:

* Ideally all extensions would use mw-ext-{extension name} as a prefix to
  their CSS classes (or something similar).

Bug: 51337
Change-Id: If4f5360cc1c7b765ad896b14901a9b024782cc93