More rigorous handling of WhatsApp error return codes

- WhatsApp sends responses of the form ((:status . 200)) (or whatever the code
  may be) whenever we perform an asynchronous operation.
- Previously, most of these were ignored. But now...
- These are represented as the STATUS-CODE-ERROR condition, which now contains a
  useful symbol that you can use to determine what type of asynchronous
  operation went wrong, and print some warning/informational message to let the
  user know that something might have not worked out.
- In cases where the status code returned relates to logging in, the LOGIN-ERROR
  subclass is used, and the connection is terminated.