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