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>

t/t7400-submodule-basic.sh

@@ -240,4 +240,11 @@ test_expect_success 'ls-files gracefully handles trailing slash' '
 
 '
 
+test_expect_success 'submodule <invalid-path> warns' '
+
+	git submodule no-such-submodule 2> output.err &&
+	grep "^error: .*no-such-submodule" output.err
+
+'
+
 test_done