[Openid-specs-fapi] Issue #373: Grant Management uses invalid_grant as authorization response error (openid/fapi)

[panva]

New issue 373: Grant Management uses invalid_grant as authorization response error
https://bitbucket.org/openid/fapi/issues/373/grant-management-uses-invalid_grant-as

Filip Skokan:

```
### Error Response

In case the `grant_id` is unknown or invalid, the authorization server will respond with an error code `invalid_grant` (as defined in [@!RFC6749]).
```

The invalid\_grant error as defined in 6749 is a “token endpoint” error.

> The authorization server responds with an HTTP 400 \(Bad Request\) status code

This specification should therefore register the error\_code for use at the different endpoints \(authorization\[, device\_authorization, ciba\]\) it is possible to return the code from in IANA.