fix: handle SerialException gracefully in all example scripts

All examples now catch serial.SerialException instead of crashing with
a traceback. This covers port contention (multiple processes), device
disconnect, and USB errors. meshcore_rx.py already had this.

simple_rx.py

@@ -9,18 +9,20 @@ Usage:
 # dependencies = ["cobs", "pyserial"]
 # ///
 
+import serial
 import sys
+
 sys.path.insert(0, __import__("pathlib").Path(__file__).parent.as_posix())
 import donglora as dl  # noqa: E402
 
-ser = dl.connect(sys.argv[1] if len(sys.argv) > 1 else None)
-print(dl.send(ser, "SetConfig", config=dl.DEFAULT_CONFIG))
-print(dl.send(ser, "StartRx"))
-
-print("\nListening (Ctrl+C to stop)...\n")
-ser.timeout = 1
-
 try:
+    ser = dl.connect(sys.argv[1] if len(sys.argv) > 1 else None)
+    print(dl.send(ser, "SetConfig", config=dl.DEFAULT_CONFIG))
+    print(dl.send(ser, "StartRx"))
+
+    print("\nListening (Ctrl+C to stop)...\n")
+    ser.timeout = 1
+
     while True:
         data = dl.read_frame(ser)
         if data is None:
@@ -30,5 +32,11 @@ try:
             p = resp["payload"]
             print(f"  RSSI:{resp['rssi']:4d}dBm  SNR:{resp['snr']:3d}dB  len:{len(p):3d}  {p.hex()}")
 except KeyboardInterrupt:
-    dl.send(ser, "StopRx")
+    try:
+        dl.send(ser, "StopRx")
+    except Exception:
+        pass
     print("\nDone.")
+except serial.SerialException as e:
+    print(f"\nSerial error: {e}", file=sys.stderr)
+    sys.exit(1)