resourceloader: Minor clean up in mw.loader and mw.html

resourceloader: Minor clean up in mw.loader and mw.html

* Remove redundant registry check in the internal
  queueModuleScript function. Like most internal methods,
  it can only be inside the mw.loader closure, and is only
  called with extant module name.

* Remove redundant registry check from execute().

* Rename local variable in mw.loader.state() from 'modules'
  to 'states' to avoid confusion.

* Shorten exception messages in mw.html to not prefix the method
  names, because we don't do that anywhere else in RL JS.

Change-Id: Idf1a49fd98445477f2d940624ca0d079325bd15b