MathJax must not process every <strong class="error">

Many things in MediaWiki (and various on-wiki templates, at least on
enwiki) output error messages wrapped in <strong class="error">. MathJax
parsing all of these (added in I1199cb34) is completely broken.

What appears to have been intended is that MathJax would parse the
errors output by MathRenderer.php. So let's add a "texerror" class to
those and have MathJax look for that class instead.

Bug: 55675
Change-Id: Iaa6c3a892af463f38e6706f9407c6dcb948fe670