[m-rev.] diff: don't stop the generation of interface files for non-error messages

[Zoltan Somogyi]

This should not need review.

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.noex
Type: application/octet-stream
Size: 1824 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20191027/97248d46/attachment-0002.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.noex
Type: application/octet-stream
Size: 21157 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20191027/97248d46/attachment-0003.obj>