dispatch: making all hg abortions be output with a specific label
This allows abortions to be highlighted specially and separately
from warnings - for instance, red is a reasonable color for when hg
aborts, but is overly dramatic for most warnings produced elsewhere.
Differential Revision: https://phab.mercurial-scm.org/D3967
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors