[ENH] - Add catches for common user mistakes

[kcpevey]

Feature description

There are some common mistakes that I think it would beneficial to check and catch before getting too far down the deployment process.

• jhsingleuserproxy if this is missing from the environment, it will take the app deployement 15 minutes or so to fail to deploy.
• missing extra args in gradio app.py files if these are missing, the app will never succeed deployment and will never fail. It will just be stuck in a perpetual deploying state (I've waited an hour and the page still couldn't load, no errors).

Value and/or benefit

Adding catches for these common gotchas will give feedback to users quicker and make the UX much nicer.

Anything else?

No response