resourceloader: Don't localise hidden exception

resourceloader: Don't localise hidden exception

Mechanical stacktraces are mostly program identifiers (which are English) with some
glue text.

When stacktraces are disabled by the site admin, the replacement message doesn't
seem valuable to localise. And one could argue it should be in the user language
instead of the site language in that case, which complicates the output buffer
and caching layers for this.

It also reduces coupling with MediaWiki (Ref T32956).

Change-Id: If446f1b815add2be4baf1c333f18a4bab0cc9c37