handle "out of space" and "quota exceeded"

Please handle "out of space" and "quota exceeded" errors with an appropriate IOErrors. What I got from duplicity was:
BackendException: sftp put of [...] (as [...]) failed: Unsupported operation

At the time I investigated, the incomplete backup had been cleaned up already (and there was quite some space left). It took me some time to identify the cause of the cryptic BackendException.