[m-rev.] diff: better error messages during module qualification

[Zoltan Somogyi]

Something I discovered the need for while working on the module_info type.

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.qual_context
Type: application/octet-stream
Size: 2683 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20150101/5288fcba/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.qual_context
Type: application/octet-stream
Size: 135484 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20150101/5288fcba/attachment-0001.obj>