[m-rev.] for review: fix undefined symbol error messages with sub-modules

[Fergus Henderson]

Simon Taylor, you wrote:
> 
> Fix a bug which caused misleading error messages when
> the module qualifier for an undefined symbol matched a
> module imported only by ancestor modules.

I will try to review this one more thoroughly later, but at first
glance it looks OK, so I suggest that you go ahead and commit it now.

-- 
Fergus Henderson              | Designing grand concepts is fun;
fjh at cs.mu.oz.au               | finding nitty little bugs is just work.
http://www.cs.mu.oz.au/~fjh   | -- Brooks, in "The Mythical Man-Month".
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