fadd069d03 (merge-recursive: give less scary messages when merge did not
start, Sep 7 2009) introduced some friendlier error message for merge
failure, but the messages were shown only for non-fast forward merges.
This patch uses the same for fast-forward.
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr> Signed-off-by: Junio C Hamano <gitster@pobox.com>