Stop converting HTTPError to ValueError in mlbgame/data.py

[dowski]

Converting HTTPError to ValueError loses valuable information about the failure. If the HTTPError itself was allowed to be raised then callers could look at the status code and determine if they want to try the call again (in the case of a 5xx error) or stop trying (4xx error).

Or maybe it could raise new exceptions defined by the mlbgame library? Fatal or Retryable?

[panzarino]

@dowski I would prefer to use a custom exception rather than a HTTPError.