Handle out of disk space failures better

The gateways have now gotten into bad states from running out of disk space a few times. Fetching content from gateways that are out of space results in something like this: https://a.uguu.se/UnPPwtbEJHS1.png

We should:
- [ ] Catch and return these errors if we encounter them.
- Randomly closing the connection on users isnt good behaviour
- [ ] Make sure that garbage collections are run frequently enough
- should also trigger a garbage collection if an 'out of disk space' error is encountered
- [ ] add tests for these conditions using either sized subvolumes (btrfs, zfs) or fixed size tmpfs mounts.

And while we're at it:
- [ ] Respect set 'repo size limitations'