[api] Inconsistency in return calls of the player endpoint

[hacketiwack]

The /api/player endpoint provides several methods for navigating through the queue. However, when the queue is empty, these methods return inconsistent HTTP response codes.

Response codes for each method and a suggested response code:

Method	Response Code	Suggested Response Code
/api/player/next	204	204
/api/player/pause	500	204
/api/player/play	500	204
/api/player/previous	500	204
/api/player/seek	500	204
/api/player/stop	204	204

[ejurgensen]

Sounds fine to me!