Treat rsync return code 24 as success

Code 24 (source files vanished during the transfer) already counted as
success when deciding whether to remove old backups, but the program
still exited with 4 and reported errors. Exit with 0 and report the
backup as completed, logging only a warning, so both checks agree.

man/simple_backup.1

@@ -180,7 +180,8 @@ for details. For this reason, use SSH key authentication if possible.
 .SH EXIT STATUS
 .TP
 .B 0
-The backup was completed without errors.
+The backup was completed without errors. This includes rsync return code 24 (some source files
+vanished during the transfer), which only logs a warning.
 .TP
 .B 1
 No valid inputs selected for backup.
@@ -192,7 +193,7 @@ Backup failed because output directory for storing the backup does not exist.
 Permission denied to access the output directory.
 .TP
 .B 4
-rsync error (rsync returned a non-zero value).
+rsync error (rsync returned a value other than 0 or 24).
 .TP
 .B 5
 SSH connection failed.