Added MediaLocation to MediaService.

There's Location type under LocationsService, the different is
the ID type. I've reported this inconsistency to Instagram
https://groups.google.com/forum/#!topic/instagram-api-developers/Fty5lOsOGEg
Handle HTTP 500 with plain message "Oops, an error occurred.".

Unfortunately this kind of error doesn't get wrapped in a typical
JSON response with meta field describing the message.