resourceloader: Don't catch LESS error in ResourceLoaderFileModule

Instead let it throw up to ResourceLoader::makeModuleResponse.. By using
the central logic for exception handling the parse error becomes more
discoverable and obvious.

* The module is marked as in "error" state (currently it would still serve
  the result of file A and C, even if file B has a parse error).
  This matches the behaviour of the javascript checker we have.
* The exception comment is produced on top of the load.php response
  (where users expect it) instead of buried somewhere between the
  different modules and the different files within the modules.
* The exception comment will not be stripped out by CSSMin.
* DRY :)

Without this, debugging less parse errors is harder than it should be.

Bug: 55442
Change-Id: I6510b43f02cf82568b7fd2421209a8e636d5e115