break out errors into explicit types

[brentru]

Addressing: #18

In particular break out connection attempt failed errors (non-zero RC in connect callback), and the disconnect errors (non-zero RC in disconnect callback) into explicit error types that inherit from a common error base. This way it's easier to catch errors like unexpected disconnects and retry connecting to the service.

Impl.: Broke out another error class for RC errors (MQTTERROR) in errors.py, made a list of errors (so we can add more into IO, #6-255 are unassigned), called in mqtt_client.py.