Build: missing submodule throws exception

[agjohnson]

If a user configures a submodule to use in their readthedocs.yaml, the build command/step to pull submodules will fail with an exception and will not report back to the user that there was a problem.

To reproduce, specify submodules.include: ['submodule-that-doesnt-exist'] in a configuration file.

Here is a test-builds test case, which failed:

https://github.com/readthedocs/test-builds/blob/submodule-not-found/.readthedocs.yaml
https://readthedocs.org/projects/test-builds/builds/17876929/

I would expect that build step to report back to the user that there is a missing submodule.

[humitos]

Looks related or duplicate of #9365

[agjohnson]

Yeah, seems related a bit. Definitely related in that the exception is not reported back to the user.

If we do anything here specifically, it could be a command/build failure that "Submodule submodule-that-doesnt-exist was not found". On #9365, that might error message might be a little different perhaps, though I could see both being covered with a generic message too.

[agjohnson]

Rolled into #9365