[m-rev.] for post-commit review: fix a misleading error message

[Zoltan Somogyi]

For review by anyone. I would like feedback on

- whether the description of the problem in the test case
  is clear enough, and

- whether anyone can think of any way to handle such
  situations better. (For example, when we find switches
  where none of the arms are reachable, we could delete
  only the arms, not the switch, but this would lead to
  an error message about the copy of the switch
  not being complete, which would be misleading
  in a different way.)

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.rcs
Type: application/octet-stream
Size: 505 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20230419/05f642a4/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.rcs
Type: application/octet-stream
Size: 7904 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20230419/05f642a4/attachment-0001.obj>