API (restart): raise exception when observer attempts to restart game

Author: feberts

client/game_server_api.py

@@ -266,6 +266,8 @@ def restart(self):
         Raises:
         GameServerError: in case the game could not be restarted
         """
+        if self._observer: raise GameServerError('cannot restart game as observer')
+
         _, err, _ = self._send({
             'type':'restart',
             'game':self._game,