Full disk during simulation time does not give a clear error

[wouterjdb]

When running FlowNet on ubuntu, there is no clear error message when your disk gets full during simulation time. ERT simply hangs or forever continues not knowing what the status is (neither is ERT killed by max runtime). If you rerun your case by overwriting the previous output folder you would remove data from the previous run and end up at the exact same problem...

We should build in some warning that there is limited disk space, this could potentially be done in here: https://github.com/equinor/flownet/blob/master/src/flownet/ert/_run_subprocess.py

This would allow for running the disk space check during simulation time (if fast enough? df -h?)