Expand `AuthenticationError`

[BURG3R5]

After #149 is merged, do the following:

• set_up_webhooks should have parameters args: dict and repository: str
• In complete_auth, args=request.args should be passed to set_up_webhooks
• In set_up_webhooks, check if "code" in args is present. If yes, then do normal stuff. If no, return whatever the value of args["error_description"] is

[Ayush0Chaudhary]

Can do it by tomorrow EOD, may have to extend deadline