If server's disk is full, Obnam seems to give no useful error message:

2015-08-29 16:31:42 ERROR Can't back up
/home/liw/ick/liw.state/extrautils/builds/106/build.log: RD5FA4X:
System error: chunks/1052/1065/3340/212da18852839223: None:
Failure

2015-08-29 16:31:42 ERROR OSError(None, 'Failure')

It looks like Obnam is getting too little info from paramiko, but maybe that can be fixed.

Reported by Bazyli Brzóska, http://listmaster.pepperfish.net/pipermail/obnam-dev-obnam.org/2015-April/000144.html