Give better feedback to users

[pameladelgado]

When a session is in a Pending state because there are not enough resources, or there is not enough disk space to start the session, or any other issue, the user has no way of knowing the cause. Currently, only admins can access this information and they need to communicate with the users in a case-by-case basis which is time-consuming and not desirable.
We need to enable this information going forward and being visible so that the user knows what could be wrong.
This together with SwissDataScienceCenter/renku-ui#787 are high priority.

[olevski]

@pameladelgado how did this work with Jupyterhub btw? What happened when we had Jupyterhub and a user tried to launch a sessions but there weren't enough resources.