User may optionally create a file rsync_failure_codes with a
newline-separated list of rsync return codes that are to be
regarded as complete failure.
If rsync exits with such a code, then the backup will be marked
for deletion during the next ccollect run (if delete_incomplete).
I added documentation for this feature in doc/ccollect.text
In my experience (yours may differ), two rsync exit codes that
belong in this file are 12 and 255. I have seen 12 result from
ssh errors and 255 result from a kernel module conflict. In both
cases, the resulting backups were empty. Without the
rsync_failure_codes option, such errors cause good backups to be
deleted, as per c_interval, leaving behind the new empty backups.
In the long run, we may want a different and more comprehensive
method for analyzing rsync errors. In the short run, I find this
option necessary.
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