Subsonic API: return error for unimplemented endpoints

While the format of getPlayQueue was wrong, had it been right, simply
returning an empty queue would have caused clients with support for
queue syncing to silently empty their play queue.

lib/Controller/SubsonicController.php

@@ -997,15 +997,15 @@ protected function deleteBookmark(string $id) {
 	 */
 	protected function getPlayQueue() {
 		// TODO: not supported yet
-		return $this->subsonicResponse(['playQueue' => []]);
+		return $this->subsonicErrorResponse(30, 'Not implemented');
 	}
 
 	/**
 	 * @SubsonicAPI
 	 */
 	protected function savePlayQueue() {
 		// TODO: not supported yet
-		return $this->subsonicResponse([]);
+		return $this->subsonicErrorResponse(30, 'Not implemented');
 	}
 
 	/**
@@ -1023,7 +1023,7 @@ protected function getScanStatus() {
 	 */
 	protected function getNowPlaying() {
 		// TODO: not supported yet
-		return $this->subsonicResponse(['nowPlaying' => ['entry' => []]]);
+		return $this->subsonicErrorResponse(30, 'Not implemented');
 	}
 
 	/* -------------------------------------------------------------------------