Submodules: user message when there are problems cloning them

[humitos]

I thought we had an issue opened for this, but I wasn't able to find it.

The problem that I've seen is that if the project is using submodules.include: all and there is at least one submodule that fails to clone for any reason the build fails with the generic message without communicating the users what's the problem.

The solution that I've told the users is to just find out what's that submodule and remove it from .gitmodules. It would be good if we could detect this scenario and tell the users

• what's the exact module that it's not working so they can remove it
• there is a submodule that will be skipped because it failed to clone

I wanted to create a test-builds branch to reproduce this case, but I'm not exactly how to create it.

[agjohnson]

I'll roll #9539 into this issue, they are related. We should throw an error for both of these cases. The error in #9539 is fairly bad UX, but shouldn't be super common -- it only affects projects that had a submodule that was partially removed.