backup: Perform preflight checks before entering the plugin loop

backup/test_backup.py

@@ -100,7 +100,7 @@ def test_start_no_init(nf, directory):
         options=opts, cleandir=False, may_fail=True, start=False
     )
 
-    with pytest.raises((RpcError, ConnectionRefusedError, ConnectionResetError)):
+    with pytest.raises(ValueError):
         # The way we detect a failure to start is when we attempt to connect
         # to the RPC.
         l1.start()