[m-rev.] for post-commit review: better error messages for arg type errors

[Zoltan Somogyi]

On Wed, 31 Dec 2014 16:56:17 +1100 (EST), "Zoltan Somogyi" <
> > "whose possible the actual"  I don't know what you mean.
> 
> I meant "whose possible actual types". Will fix.

This is that fix.

> (Note that in the presence of type errors, we don't know what the type
> of a variable IS; we only know what it MAY BE.

> > Should this be in the other file with subsumes_type?

It also improves some code that deals with the type subsumption
predicate referenced here.

Zoltan.

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