HTTP 500 authentication error in CIBA workflow when user deny request

Bug description

When user Deny request from CIBA, there is HTTP 500 error instead of 403 access_denied

How to reproduce the issue

1. follow https://docs.google.com/document/d/1HlPCIUDZj7uQsMXOTie_hSGzE3qaS0Rn4GJg_1KKfYc/edit to configure CIBA
2. do CIBA workflow and when notification come to your phone, do CIBA polling request, there is "slow down" or "pending" status
3. deny request on your phone
4. do polling request

Expected behaviour

Response should be HTTP 403 access_denied

Current behaviour

There is HTTP 500 

{
    "error": "server_error",
    "error_description": "Error occurred during authentication"
}

with no output in debug on debug=error