Catch and report invalid credentials error

[SpainTrain]

Overview

Currently one can provide no credentials, or invalid credentials, to this library and there is not error until CM API responds with 403. Even then, the cause of the error isn't clear (in the consuming app, given an app could have 403s for a number of reasons).

Add some basic handling around invalid credentials.

Tasks

• Handle and throw descriptive error when creds aren't passed to lib
• Handle and throw descriptive error when invalid creds are passed

Acceptance Criteria

As a consumer of this library, if I pass invalid creds it is either immediately obvious upon initing the lib, or it is clear when I make the first call the a CM API endpoint.