Separate ErrorResponse from response types

[rj-calvin]

After some development with fetch on the frontend, I've come to the conclusion that it is better for me to assume that the responses are always valid until they are not. In other words, I would prefer if the various response types do not union with ErrorResponse and instead just be defined as the data that is relevant to a successful request. Dealing with the ambiguity of which is which is something that I can handle on the frontend using a bit of logic and type casting.