submodule--helper: offer a consistent API

submodule--helper: offer a consistent API

In 48308681 (2016-02-29, git submodule update: have a dedicated helper
for cloning), the helper communicated errors back only via exit code,
and dance with printing '#unmatched' in case of error was left to
git-submodule.sh as it uses the output of the helper and pipes it into
shell commands. This change makes the helper consistent by never
printing '#unmatched' in the helper but always handling these piping
issues in the actual shell script.

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