resourceloader: Fix testMakeModuleResponseError() failure on Travis

resourceloader: Fix testMakeModuleResponseError() failure on Travis

> 1) ResourceLoaderTest::testMakeModuleResponseError
> Failed asserting that '[e08c982d974548127cb5d7ce] Fatal exception of type Exception'
> matches PCRE pattern "/Ferry not found/".
> .../ResourceLoaderTest.php:519

This happened on Travis CI, because ResourceLoader::formatException() behaves
differently based on $wgShowExceptionDetails. Which is enabled in Vagrant
and Jenkins, but disabled by default (and thus in Travis CI builds).

Bug: T75176
Change-Id: If15dd03213703b7b6ff899cad5e5569e2515b378