Use the current proxy as fallback when failing to fetch a new one

Prevents accidentally resetting the proxy!

Author: Fizzadar

mautrix/util/proxy.py

@@ -51,11 +51,10 @@ def get_proxy_url_from_api(self, reason: str | None = None) -> str | None:
                 response = json.loads(f.read().decode())
         except Exception:
             self.log.exception("Failed to retrieve proxy from API")
+            return self.current_proxy_url
         else:
             return response["proxy_url"]
 
-        return None
-
     def update_proxy_url(self, reason: str | None = None) -> bool:
         old_proxy = self.current_proxy_url
         new_proxy = None