Solve problem of users unable to start sessions due to full arc home directory

[chriswillott]

When a user has filled their home directory (default quota 200GB) they are unable to start new sessions and have no feedback to know that this is the problem. This is happening quite frequently and is a drain on CANFAR support as people have to investigate to discover the issue.
Possible work to improve the user experience:

1. Provide new users with a FAQ containing the home dir quota information, plus other other useful information including that large files are typically to be placed in arc projects directories that have much higher quotas.
2. Warn users when they are at 90(TBD)% of their home dir quota so they know that they need to clean up and keep below the limit.
3. Provide a useful error message about the full home directory when they try and fail to start a session.

[sefffal]

I hit a similar issue with a full project directory recently that resulted in some lost files. If warnings are implemented may I also request that they are generated for project directories?
Thanks!