Addressing 502 errors

I've noticed quite a few 502 errors from the Metadata API. It seems to be a problem on Spotify's end. I added a condition to properly catch these 502 errors.

I also switched the query string to encode as form data -- basically using "+" instead of "%20" to represent spaces in the query. This conforms to [Spotify's guidelines][1] and in my experience has helped reduce the number of 502's from the API.

Thanks for your time and work thus far!

[1]: https://developer.spotify.com/technologies/web-api/