submodule: warn about non-submodules

Earlier, when you called

        git submodule some/bogus/path

Git would silently ignore the path, without warning the user about the
likely mistake.  Now it does.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/git-submodule.sh b/git-submodule.sh
index 2f47e06..6cc2d33 100755
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -59,7 +59,7 @@
 #
 module_list()
 {
-	git ls-files --stage -- "$@" | grep '^160000 '
+	git ls-files --error-unmatch --stage -- "$@" | grep '^160000 '
 }
 
 #