failure to fetch git submodules are not errors

[kit-ty-kate]

c030dcb (in opam 2.0) added support for git submodules when pulling a source repository.
However currently failures while fetching the submodules are not errors but only displays a warning. I think this should be an error instead, as not stopping early can change the behaviour of building a such a package drastically.

[hhugo]

related to #6021

[kit-ty-kate]

good point, although in your case Windows' MAX_PATH is the real culprit but indeed failing early would show a better error message for sure