[ENH] - Add support for failed apps debugging

[aktech]

Feature description

As of now when an app fails to load, the JupyterHub throws a 404, without any error message, it will be great to show exact error instead.

Value and/or benefit

Users being able to debug issues with their apps.

Anything else?

cc @tylerpotts

[tylerpotts]

Only other thing is to investigate the timeout for apps that take a while to start. It may be the case that the timeout isn't the issue when we're able to see logs, so not a hard requirement