Internationalisation of error handling

[jawrainey]

Currently, when we raise a CustomException we manually write a "reason" in the response. This is not ideal as these are in English whereas localisation should occur on the clients of the consumer API (patient and clinical apps). As such, we should ideally return a set of pre-defined keys that clients can use to lookup appropriate messages to send to users, for example, INVALID_TOKEN.

I suggest that we add an enum (named ResponseErrors) to errors.py and modify CustomException so that it holds a list of ResponseErrors.

As an example, the inventory service would change from:

if res.status_code == 429:
        raise CustomException(errors=["Rate Limit on API access."], status_code=429)

to:

if res.status_code == 429:
        raise CustomException(errors=[ResponseErrors.RATE_LIMIT], status_code=429)