Blender Git Loki

make_update: Fix case where a sub-module would not have required branch.

Issue revealed by rB546314fc9669 change, also error itself exited before
that commit.

Now we do accept git command to fail when trying to checkout the
specified branch from sub-modules, and only actually error in case the
fall-back branch (aka master) cannot be properly checked out.

Thanks fot Ray molenkamp (@LazyDodo) for report and initial patch
(D12560).