diff/status: print submodule path when looking for changes fails

diff/status: print submodule path when looking for changes fails

diff and status run "git status --porcelain" inside each populated
submodule to see if it contains changes (unless told not to do so via
config or command line option). When that fails, e.g. due to a corrupt
submodule .git directory, it just prints "git status --porcelain failed"
or "Could not run git status --porcelain" without giving the user a clue
where that happened.

Add '"in submodule %s", path' to these error strings to tell the user
where exactly the problem occurred.

Reported-by: Seth Robertson <in-gitvger@baka.org>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>