backup: fix borg exit code handling for ret=0

backup.py

@@ -379,7 +379,7 @@ def main(argv: list[str]):
             backup.log('E', f"borg exited with errors (ret={ret})")
         elif ret == 1 and borg_saw_warnings:
             backup.log('W', f"borg exited with warnings (ret={ret})")
-        else:
+        elif ret != 0:
             backup.log('E', f"borg exited with unknown error code {ret}")
 
     # See if we had any errors