sftp backend: falsely reports ok if target fs was r/o

[az]

this is a copy of debian bug #596857, which lives here: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=596857

synopsis: when the target fs is mounted read-only and you're using sftp, then duplicity claims success for
backups even though it couldn't save a single bit.

this somehow happens only if the fs is mounted r/o - "normal" insufficient permissions result in
a misleading error message ("Invalid SSH password") and lots of retries.

if you use --use-scp, you get lots of retries and 'scp failed' errors.

the original submitter reported this for 0.6.08 and i've confirmed it for 0.6.09.