Emit specific error if protocol modules not found

This has now tripped up more than one user (see issues #47 and #53). There is a note in the README about this, but it's easy to miss if you don't read it thoroughly.

dump_dish_status.py

@@ -1,11 +1,19 @@
 #!/usr/bin/python3
 """Simple example of get_status request using grpc call directly."""
 
+import sys
+
 import grpc
 
+try:
     from spacex.api.device import device_pb2
     from spacex.api.device import device_pb2_grpc
     from spacex.api.device import dish_pb2
+except ModuleNotFoundError:
+    print("This script requires the generated gRPC protocol modules. "
+          "See README file for details.",
+          file=sys.stderr)
+    sys.exit(1)
 
 # Note that if you remove the 'with' clause here, you need to separately
 # call channel.close() when you're done with the gRPC connection.