[m-rev.] for post-commit review: improve diagnostics for mode merge errors

[Zoltan Somogyi]

For post-commit review by anyone.

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.merge
Type: application/octet-stream
Size: 657 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20200701/b694e9cc/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.merge
Type: application/octet-stream
Size: 17917 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20200701/b694e9cc/attachment-0001.obj>