[m-rev.] for review: better error messages for lambda exprs

[Zoltan Somogyi]

For review by anyone. If possible, I would like a review of the
change to NEWS by more than one person.

Like several of my diffs in the last few months, this one improves
the error messages we generate for syntax errors, but restricts
the set of legal Mercury programs. If anyone objects to the restrictions
imposed by this diff, please speak up.

Zoltan.
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