Handle missing/ bad/ expired/ etc.. authorization tokens

[WilliamMcCumstie]

If the jwt_token field is missing within the config the following is returned. Unfortunately the exact same error will be issued if the token is syntactically invalid or expired.

# bin/flight-cu balance
flight-cu: Unrecoverable server-side error encountered (500)

As setting the jwt_token is document in etc/config.yaml.reference, specific error handling for missing tokens is not required. Due to the 500 status code, it is not possible to determine exactly why a token isn't valid.

This issue is on hold until the 500 is fixed to either 401 or 403 (maybe 400/404) as appropriate.