Fix last_invalid_packet_exception logging

Ensure the last_invalid_packet_exception parameter contains the datatypes expected by StatisticsConsumer

Closes L2SS-936

lofar_station_client/statistics/collector.py

@@ -59,7 +59,7 @@ class StatisticsCollector(abc.ABC):
             # Full contents of the latest packet we deemed invalid.
             "last_invalid_packet": numpy.zeros((9000,), dtype=numpy.uint8),
             # Track the last invalid packet
-            "last_invalid_packet_exception": None,
+            "last_invalid_packet_exception": "None",
         }
 
     def process_packet(self, packet):
@@ -75,7 +75,7 @@ class StatisticsCollector(abc.ABC):
             )
             self.parameters["nof_invalid_packets"] += numpy.uint64(1)
 
-            self.parameters["last_invalid_packet_exception"] = err
+            self.parameters["last_invalid_packet_exception"] = str(err)
 
             raise ValueError("Could not parse statistics packet") from err