Define error for failed token fetch in TokenTransport

[oszkar-jona-form3]

As a user of the TokenTransport,
I want to explicitly check if the returned error is caused by failed token fetch
so that I can execute a different branch of code.

Currently, a generic error message is returned:

go-form3/pkg/form3/auth_token.go

Line 108 in 9d40a2d

return nil, fmt.Errorf("non 200 status code getting token, status code: %d", authResponse.StatusCode)

Desired outcome would be to have a defined error (or custom error type should work as well)

var ErrGetAuthToken = errors.New("error getting auth token")