Currently the editor deletes its temp file whenever editing is finished.
With this patch, the file will not be deleted if the editor callback
encounters an exception.
One example is if the tab containing the edited element is closed. The
editor errors with "Edited element vanished", but with this patch it
will also print "Backup at ..." so the user does not lose their work.
Resolves#1596.
Supersedes #3641, using the cleaner approach started in #1677.
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