Renku 2.0: launching a session with a non-existent image gets the UI stuck

[leafty]

Reproduce:

1. Create a project in Renku 2.0
2. Create a new launcher in the project and use a custom image with a dummy value, e.g. renku/image-that-does-not-exist
3. Launch session

I believe this is probably a leftover from Renku 1.0 where the UI would wait for the image to be available (with a timeout) if it is being built by CI/CD in GitLab.