Fix error message serialization

[dsbankov]

I guess musixmatch changed the error message response recently. An error message response currently looks like this:

{
	"message": {
		"header": {
			"status_code": 404,
			"execute_time": 0.0067391395568848
		},
		**_"body": []_**
	}
}

The "body" element was previously serialized as a string which caused an IllegalStateException to be thrown when an HTTP 404 status was returned (for example when we're trying to get a track with an inexisting ID).