mw.loader.store: Count modules which fail to execute, too

mw.loader.store: Count modules which fail to execute, too

"Failed" will probably always be either zero or one, because it is
unlikely the asyncEval error callback can ever be called more than once.
But the stats will let us verify this, and will give us an indication of
how often errors happen in general.

Change-Id: I6dea28d84bbca0f02ad8c5bf9a8e9d976622d815