Attempt to disconnect MQTT cleanly if error occurs during upload

Have monitored different errors from MQTT during upload after connection.  In the event of an OSError or IndexError try to disconnect MQTT to see if it prevents hangs.

enviro/destinations/mqtt.py

@@ -36,6 +36,19 @@ def upload_reading(reading):
     mqtt_client.disconnect()
     return UPLOAD_SUCCESS
 
+  # Try disconneting to see if it prevents hangs on this typew of errors recevied so far
+  except (OSError, IndexError) as exc:
+    try:
+      buf = io.StringIO()
+      sys.print_exception(exc, buf)
+      logging.debug(f"  - an exception occurred when uploading.", buf.getvalue())
+      mqtt_client.disconnect()
+    except Exception as exc:
+      import sys, io
+      buf = io.StringIO()
+      sys.print_exception(exc, buf)
+      logging.debug(f"  - an exception occurred when disconnecting mqtt client.", buf.getvalue())
+
   except Exception as exc:
     import sys, io
     buf = io.StringIO()