fix: pre-probe MQTT port for clear error on blocked networks (#107)

When port 8883 is blocked by campus VPNs or firewalls (e.g. eduroam),
the MQTT client would hang for 60s then crash with a raw traceback.

Now pre-probes the port with a 5-second socket connection test:
- On failure: prints a clear error message suggesting the network
  may be blocking the port, then exits gracefully.
- Also fixes a crash where setsockopt was called on a None socket
  after a failed MQTT connect (the except block continued execution
  instead of exiting).

Closes #107

Author: mwfarb

arena/arena_mqtt.py

@@ -225,10 +225,27 @@ def __init__(
         else:
             self.transport.tls_set_context(ssl._create_unverified_context())
             self.transport.tls_insecure_set(True)
+        # pre-probe MQTT port to fail fast with a clear error on blocked networks
+        probe_timeout = 5  # seconds
+        try:
+            probe = socket.create_connection((self.mqtt_host, port), timeout=probe_timeout)
+            probe.close()
+        except (socket.timeout, OSError) as err:
+            print(f"\n{'='*60}")
+            print(f"ERROR: Cannot reach MQTT broker at {self.mqtt_host}:{port}")
+            print(f"  Connection failed: {err}")
+            print(f"")
+            print(f"  This commonly happens when your network blocks port {port}")
+            print(f"  (e.g. campus VPN, eduroam, corporate firewall).")
+            print(f"  Try connecting from a different network.")
+            print(f"{'='*60}\n")
+            sys.exit(1)
+
         try:
             self.transport.connect(self.mqtt_host, port=port, keepalive=60)
         except Exception as err:
-            print(f'MQTT connect error to {self.mqtt_host}, port={port}: Result Code={err}')
+            print(f"MQTT connect error to {self.mqtt_host}, port={port}: {err}")
+            sys.exit(1)
         self.transport.socket().setsockopt(socket.SOL_SOCKET, socket.SO_SNDBUF, 2048)
 
     def generate_client_id(self):