Improve error output

[bhelm]

Here is the error i had with btrfs-sxbackup. Can you tell the cause?

root@ns320047:~/btrfs-sxbackup# /usr/bin/flock -n /tmp/mariadb.lockfile /usr/bin/python3 /usr/local/bin/btrfs-sxbackup run /backup/mariadb/
INFO btrfs-sxbackup v0.5.8
INFO source :: url [ssh://root@172.16.0.63:/home/mysql/] container [.sxbackup/] retention [1] compress [True]
INFO destination :: url [/backup/mariadb/] retention [2w:4/d, 1m:daily, 3m:none] compress [True]
INFO preparing environment
INFO source :: retrieving snapshots
INFO destination :: retrieving snapshots
INFO source :: creating snapshot
INFO source :: transferring snapshot
INFO destination :: removing subvolume [/backup/mariadb/.temp.e1a73f6bf3104420bb36c353ca47572a]                                                                                                        <=>                                                                                                                  ]
INFO source :: removing subvolume [/home/mysql/.sxbackup/.temp.e1a73f6bf3104420bb36c353ca47572a]
ERROR Command '['ssh', '-o', 'ServerAliveInterval=5', '-o', 'ServerAliveCountMax=3', 'root@172.16.0.63', 'btrfs send "/home/mysql/.sxbackup/.temp.e1a73f6bf3104420bb36c353ca47572a" | lzop -1']' returned non-zero exit status -9
ERROR At subvol /home/mysql/.sxbackup/.temp.e1a73f6bf3104420bb36c353ca47572a
2,49GiB 0:01:14 [34,3MiB/s] [                                                                                                                                                               <=>                                                                                                                             ]
root@ns320047:~/btrfs-sxbackup# 

The backup failed because of no free disk space on the receiving volume. Looking into the source, btrfs-sxbackup checks the sending return code first and throws the error above. in the "no free disk space on the receiving partition" case, it seems to me that send and receive side fail at the same time. btrfs receive prints an error, but btrfs-sxbackup only shows stderr on the sender side.

btrfs-sxbackup should report errors on both sender and receiver side if possible.