merge-ort: upon merge abort, only show messages causing the abort

merge-ort: upon merge abort, only show messages causing the abort

When something goes wrong enough that we need to abort early and not
even attempt merging the remaining files, it probably does not make
sense to report conflicts messages for the subset of files we processed
before hitting the fatal error.  Instead, only show the messages
associated with paths where we hit the fatal error.  Also, print these
messages to stderr rather than stdout.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>