resourceloader: Remove "expected error" noise from mw.loader tests

resourceloader: Remove "expected error" noise from mw.loader tests

After commit bbded40a2b26c, the stub for mw.track no longer worked
as originally intended, causing the stack traces to be printed
to the Karma output and browser console again.

The test still passed because mw.trackError calls mw.track, but
it no longer stubbed out the trace logging noise.

Also simplify some assertion messages while at it, these are meant
to describe the subject (the compared value) not be the value
itself (which is the second param) and not the error message
(which QUnit produces).

Change-Id: I657bf4479a40e1b73b45426df98882aed32b4cba