Post a warning when cloning a git submodule fails

[olevski]

Related to #1466.

Should be done after #1466 and after #1449.

Git submodules are often mis-configured. We have turned off failing the session launch if a git submodule cannot be loaded properly. But we should post a warning similar to what we plan in #1449.

It will be interesting to see how argo workflows or similar solutions surface errors/warnings from their containers into the CRD status. That is more or less what we are trying to do but in a simpler manner.

EDIT: See how feasible it is to implement something like what argo does to extract errors and warning from all containers in the jupyter server and save them in the status field in the JupyterServer CRD.

[Panaetius]

There's not really a good way to solve this in a k8s way, as pods don't really have a way to communicate back to an operator and can't treat different exit_codes differently.

We should table this until we have a notification message queue in the UI and then just send notifications to the user directly using that.