Log an error for every failed attempt to connect to master

The connection timeout and number of attempts are hardcoded, so a failure will take very long
These log lines will allow to troubleshoot issues with the connection to master

locust/runners.py

@@ -1336,6 +1336,10 @@ def connect_to_master(self):
         self.client.send(Message("client_ready", __version__, self.client_id))
         success = self.connection_event.wait(timeout=CONNECT_TIMEOUT)
         if not success:
+            if self.retry < 3:
+                logger.debug(f"Failed to connect to master {self.master_host}:{self.master_port}, retry {self.retry}/{CONNECT_RETRY_COUNT}.")
+            else:
+                logger.warning(f"Failed to connect to master {self.master_host}:{self.master_port}, retry {self.retry}/{CONNECT_RETRY_COUNT}.")
             if self.retry > CONNECT_RETRY_COUNT:
                 raise ConnectionError()
             self.connect_to_master()