Zeus throws objects as errors

[smcgivern]

If a response from the server isn't OK, handleFetchResponse will reject the promise (throwing an error) with the JSON-parsed version of the response:

graphql-zeus/packages/graphql-zeus-core/TreeToTS/functions/new/apiFetch.ts

Line 10 in cfd6d58

reject(JSON.parse(text));

This seems like it would either be better off as a GraphQLError, or a different type. Throwing an object as an error is a bit confusing for callers (see graphql/graphql-js#1393, for example).