send-email: fix missing error message regression
Fix a regression with the "the editor exited uncleanly, aborting
everything" error message going missing after my
d21616c0394 (git-send-email: refactor duplicate $? checks into a
function, 2021-04-06).
I introduced a $msg variable, but did not actually use it. This caused
us to miss the optional error message supplied by the "do_edit"
codepath. Fix that, and add tests to check that this works.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>