Decode API errors

[nineinchnick]

APIs return errors as JSON objects, usually with an error code and a message. Retrofit2 that we use for making API calls doesn't deserialize these but they're available in the response.errorBody().

Add an error model and a method to deserialize errors so the message can be made visible to the user.

Refs:

• https://docs.github.com/en/rest/overview/resources-in-the-rest-api#client-errors

[nineinchnick]

I did this only for artifacts in 46b68b1