submodule update: continue when a clone fails

submodule update: continue when a clone fails

In 15ffb7cde48 (2011-06-13, submodule update: continue when a checkout
fails), we reasoned it is ok to continue, when there is not much of
a mental burden by the failure. If a recursive submodule fails to clone
because a .gitmodules file is broken (e.g. :
fatal: No url found for submodule path 'foo/bar' in .gitmodules
Failed to recurse into submodule path 'foo', signaled by exit code 128),
this is one of the cases where the user is not expected to have much of
a burden afterwards, so we can also continue in that case.

This means we only want to stop for updating submodules in case of rebase,
merge or custom update command failures, which are all signaled with
exit code 2.

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