[MLton] Implementing warnExnMatch

[Stephen Weeks]

> I definitely get an error:
...
> Whether it gives an error or a warning appears to be under the control of
> Control.MC.matchRedundant{Error,Warn}.
...
> But, I believe the default behavior is to error.

Ah yes.  I see that I have the following in my SML/NJ startup file so
that it follows the Definition.

	      bindNonExhaustiveWarn := true
	      ; matchNonExhaustiveWarn := true
	      ; matchNonExhaustiveError := false
	      ; matchRedundantWarn := true
	      ; matchRedundantError := false

Sorry for the confusion.