Fix error message if message handler fails

There are a number of situations in which we log a message if an
exception occurs during the handling of a message:

  1) Something goes wrong pulling the message from the queue and
     de-serializing it - here we print "Failed to process message"

  2) An RPC endpoint method raises an expected exception - here we
     print an 'Expected exception during message handling' debug
     message

  3) An RPC endpoint method raises any other exception - here we
     should print an 'Exception during message handling' error message

However, in the latter case, we are currently printing out the 'Failed
to process' error message.

Change-Id: I4f7042b8ec978aaff8f4e20e62ba1ac765fe6ba5