Runtime predicate errors in forall/exists/iota/comprehensions mislocated

[nickbattle]

If a value exception occurs while processing the predicate part of a forall, exists, iota or a comprehension expression, the location of the error is given as the location of the overall expression rather than the predicate. This is confusing! For example, passing {nil} to this function:

	f: set of [bool] -> bool
	f(s) ==
		exists e in set s &     <--- Debugger stops at this line
			e;              <--- Error really occurs here

It's not critical, but it would make debugging less confusing if this was fixed.

[nickbattle]

Fix now available in ncb/development.