Add test to cover authentication failure.
Refactor session code.
Implement Account access.
Check for authentication token (broken test).
Refactor extract constants.
Change abstraction layer over HTTP requests.

Instead of having our own httpClient interface with one method for each HTTP verb we need, we can build on the http.RoundTripper interface and have more control over requests.
Send auth token on request headers (pass all tests).
Explicitly check for an authentication error.