method call return null no longer gives bogus error.

Previously, if you had this:
    // FOO IS some sort of list.
    SET ITER TO FOO:ITER.

And did this:
    ITER:RESET.

Instead of this:
    ITER:RESET().

Then it *would* work, but after calling Enumerator.GetMethod("RESET")
it would incorrectly claim "RESET" is not a suffix of ITER.  This was
because Structure.GetMethod() uses null returns as its way of
communicating that a suffix doesn't exist.  It should have used an
exception, which is what it does now.
NUnit tests changed for new GetMember exceptions.

Some of the Nunit tests failed because they expected GetMember to
return null (like it used to) on missing suffixes instead of
throwing an exception on missing suffixes.