Proper error handling required

[Chappie74]

Describe the bug An error is thrown when trying to get a new access token using refresh_token grant of a revoked token.

To Reproduce Steps to reproduce the behavior:

1. Authenticate using /auth/token (password grant), get a refresh_token and an access_token
2. Revoke the access token using auth/revoke-token.
3. With the refresh token generated in 1. , try to get a new access token using /auth/token (refresh_token grant)
4. This will throw 500 error, oauth2_provider.models.AccessToken.DoesNotExist: AccessToken matching query does not exist.

Expected behavior I expect the error to be handled silently, and a probably return 400 status code, with message invalid grant.

Screenshots