thumb.php: Set proper output formats for messages going into HTML

Authored by Legoktm.

Description

thumb.php: Set proper output formats for messages going into HTML

  • Use ->parse() instead of ->text() for wikitext messages that were being treated as HTML
  • Explicitly specify ->parse() if no output format was set
  • Document that wfThumbError() takes HTML

Bug: T76686
Change-Id: Id6e7548b2e081cfda7803772ed0395a15feb1f84