Fix unhelpful error msg when serial cannot connect.

Author: Terracom12

gps/src/gps_driver/gps_driver/rtcm_publisher.py

@@ -37,6 +37,7 @@ def __del__(self):
 
     def try_connect_serial(self):
         self.stream = None
+        self.rtcm_reader = None
 
         # Try to iterate through the three USB ports in use
         # Assume that ublox_gnss modules have already taken control of other ports