Correct error handling for exceptions in 'user' module

Correct error handling for exceptions in 'user' module

Rearrange code so that the try...catch which is supposed to catch
exceptions when evalling code actually catches them. Evaluation of
'user' module was wrapped in `mw.loader.using( 'site' ).always( ... )`,
so it could be executed asynchronously, so try...catch never caught
exceptions from it; they bubbled up to all kinds of weird places and
broke things in confusing ways.

I think the same issue could occur for any module when waiting for
legacy modules to load ('wgResourceLoaderLegacyModules').

Bug: T145970
Change-Id: I91e7d0b4e50c786f7302e30a2b7ed43c3cd0da6c