Make ResourceLoader error formatting the same as everywhere else.
Which means if wgShowExceptionDetails is enabled locally, the
trace will be included as well.
This matches logic in MWExceptionRenderer.
Also move the typical error handling used by respond() to a
utility method to reduce duplication of code and avoid mistakes.
Change-Id: If04ae99618e4a758ed0f9dd2b555496b76da29de
