Custom error raising method

https://gitlab.com/meltano/sdk/-/blob/main/singer_sdk/authenticators.py#L294

Only checks the response code to see if the request is a success.

The api I'm working with will respond with a 200, and a body of

{
    "error": "invalid_code"
}

When there's an error.

Right now the error I'm given is

    self.access_token = token_json["access_token"]
KeyError: 'access_token'

As there's no access token available in the response.

At the end of the day I'd like to see the Error here when this fails, lots of ways to get there