rerere: error out on autoupdate failure

rerere: error out on autoupdate failure

We have been silently tolerating errors by returning early with an
error that the caller ignores since rerere.autoupdate was introduced
in v1.6.0-rc0~120^2 (2008-06-22).  So on error (for example if the
index is already locked), rerere can return success silently without
updating the index or with only some items in the index updated.

Better to treat such failures as a fatal error so the operator can
figure out what is wrong and fix it.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>