Handle "tex-error" spans

[gnprice]

Some messages' content can have an HTML element that looks like this (sample message):
<span class="tex-error">$${$$</span>

We should handle those.

I believe in particular this happens when someone writes a message with markup for a LaTeX math span or block, but their LaTeX code has an error.

This is a low-priority issue because I believe these are rare. It was discovered in the #917 / #190 survey of content features not yet implemented in this app.

Related issue (but not a prerequisite, nor vice versa):

• TeX in the message list #46