NFS creation from custom image fails silently

[julianhess]

If the user specifies an invalid image/project, or does not have permission to view the specified image, the backend will hang forever at "Waiting for NFS to be ready …"

[agraubert]

Should be an easy fix. We can just run a permissions check during __init__ or __enter__

[julianhess]

Also add a timeout — if it takes longer than ~5-10 minutes at this step, abort.

We could warn the user early that the operation is taking abnormally long after ~1 minute.