submodule init: fail gracefully with a missing .gitmodules file

submodule init: fail gracefully with a missing .gitmodules file

When there is no .gitmodules file availabe to initialize a submodule
from, `submodule_from_path` just returns NULL. We need to check for
that and abort gracefully. When `submodule init` was implemented in shell,
a missing .gitmodules file would result in an error message

    No url found for submodule path '%s' in .gitmodules

Replicate that error message for now.

When the .gitmodules file is missing we can probably fail even earlier
for all of the submodules with an improved error message.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>