Added code to genjs.grace to do better module-name tracking.
Also, added comments to emitted setLineNumber code to try to find out why
spurious line-number resets are being generated.

In gracelib.js, added a try-finally around methdocalls, so that the callstack
will be unwound when a method returns, even if it raises an exception.
(The full stack will have bee copied into the exception packet before this
happens.)

In minigrace.in.js, clarified that a complier crash is diffeent from a
Grace RuntimeError