Hard fail on bad server-string (#6086)

* If server-string can't be parsed, fall back to localhost.

Co-Authored-By: Luke Childs <lukechilds123@gmail.com>

Co-authored-by: ghost43 <somber.night@protonmail.com>

electrum/network.py

@@ -268,8 +268,8 @@ class Network(Logger):
             try:
                 deserialize_server(self.default_server)
             except:
-                self.logger.warning('failed to parse server-string; falling back to random.')
-                self.default_server = None
+                self.logger.warning('failed to parse server-string; falling back to localhost.')
+                self.default_server = "localhost:50002:s"
         if not self.default_server:
             self.default_server = pick_random_server()