Change: Return exit code 3 when failing to backup all source data

The backup command used to return a zero exit code as long as a snapshot
could be created successfully, even if some of the source files could not
be read (in which case the snapshot would contain the rest of the files).

This made it hard for automation/scripts to detect failures/incomplete
backups by looking at the exit code. Restic now returns the following exit
codes for the backup command:

 - 0 when the command was successful
 - 1 when there was a fatal error (no snapshot created)
 - 3 when some source data could not be read (incomplete snapshot created)

https://github.com/restic/restic/pull/2546
https://github.com/restic/restic/issues/956
https://github.com/restic/restic/issues/2064
https://github.com/restic/restic/issues/2526
https://github.com/restic/restic/issues/2364