coccicheck: fail "make" when it fails

coccicheck: fail "make" when it fails

With "make coccicheck", we generate contrib/coccinelle/*.cocci.patch
files that contain changes suggested by semantic patches, but "make"
succeeds.  Admittedly, not many developers may run "make coccicheck"
in the first place, but it makes it harder to notice when they do
run it after they introduced an iffy piece of code.

Check that the resulting cocci.patch files are all empty.

Signed-off-by: Junio C Hamano <gitster@pobox.com>