Make HTMLForm::formatErrors non-static to can parse message in context

One call in core already called it non-static

Avoid:
[GlobalTitleFail] MessageCache::parse called by
Message::toString/Message::parseText/MessageCache::parse with no title
set

Change-Id: Ic91e715177c0a4578825640a31ec68ecba3176e0
Merge "Make HTMLForm::formatErrors non-static to can parse message in context"