From the discover tools, media().download(...) is triggering a request to https://youtubereporting.googleapis.com/v1/media/test?alt=json which is returning a 400 for me with the following content:

[{
  "error": {
    "code": 400,
    "message": "Invalid value for query parameter 'alt'. It must be set to \"media\".",
    "errors": [
      {
        "message": "Invalid value for query parameter 'alt'. It must be set to \"media\".",
        "domain": "global",
        "reason": "badRequest"
      }
    ],
    "status": "INVALID_ARGUMENT"
  }
}
]

The contents of the response are put into an HttpError in HttpRequest.execute but the _get_reason expects {"error": ...} directly instead of [{"error": ...}] so it is crashing with

<type 'str'>: (<type 'exceptions.TypeError'>, TypeError('list indices must be integers, not str',))

I'm not sure if there is a valid reason to have a list of errors and the client code should support it or if the server code should return a single error with the "errors" subelement filled in (as it is).