Display login errors on NLX

[gdestuynder]

Currently login errors are sent to the SSO dashboard in the GET parameter, using a JWT.

We would like to send the errors to NLX using a GET parameter, with state verification instead, because this allows for a better UX in NLX.

Not only is this faster and more consistent, but it also allows NLX to act on the error message in different ways.

Since NLX is currently hosted by auth0, the state verification works out of the box. What's needed is the ability to ingest and display messages.

Proposal:

/login?nlx_error=base64encodedmsg&state=encodedString

where NLX would display the message.

The current list of messages are: https://github.com/mozilla-iam/sso-dashboard/blob/master/dashboard/auth.py#L129-L165

This work is part of the LoginV2 efforts.

cc @mbransn @hidde @gene1wood

[gene1wood]

Would the GET parameters also include state? Such that it would look like

/login?nlx_error=base64encodedmsg&state=encodedString

[gdestuynder]

yes, it has to