Return apropriate error and exit code when backup file is too large

There is a backup size limit on the server side. When the backup is too large, ssbackups client exits with 255 error code and print json to stdout:

root@omnia: ~ # ssbackups create bigfile.bak
Password for backup: 
Password for backup (again): 
{"type": "ClientException", "detail": "Command << ['/usr/bin/curl', '--fail', '-X', 'POST', '-m', '10', '-H', 'Accept:application/json', '-H', 'Authorization:Token 01234DEADBEEF', '-F', 'payload=@bigfile.bak', 'https://rb.turris.cz/backups/'] >> ended with << return code = 22 >> and with output <<  >>.", "exit_code": -1}

Foris wrongly reports "Failed to connect to the ssbackup server." then (see screenshot).

ssbackups cli client should report some apropriate message on stderr and return defined exit code
Foris should report similar error message instead of general "Failed to connect..."
A notification message should be raised when (regular timed) backup was not finished successfully, so the user get noticed that the backups are not created periodically

cc: @mmladek @shenek

Edited May 30, 2018 by Michal Mladek