fetch: emphasize failure during submodule fetch

fetch: emphasize failure during submodule fetch

In cases when a submodule fetch fails when there are many submodules, the error
from the lone failing submodule fetch is buried under activity on the other
submodules if more than one fetch fell back on fetch-by-oid. Call out a failure
late so the user is aware that something went wrong, and where.

Because fetch_finish() is only called synchronously by
run_processes_parallel, mutexing is not required around
submodules_with_errors.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>