resourceloader: Improve coverage of ResourceLoader::respond()

- Cover case of simple module load.
  The bulk of this use case is already covered by a lower-level
  test for makeModuleResponse(). The added case here exists
  to cover the wrapper method, ResourceLoader::respond().

- Cover logic for catching and logging internal errors.

Change-Id: I4315bb00137ff80ee2b790c6b4d4b5fbd93f6bc1