Simulator error checking

[jrbaxter0]

This bug leads to cryptic errors when the cause if very straightforward. Rather than crashing and giving a convenient error message when a necessary parameter wasn't given, the simulator skips to the next section of code.

This same practice may exist in several files in rktl_sim

Before:

        # Setting up ball
        ball_img_path = rospy.get_param("~BALL_IMG_PATH", None)
        if ball_img_path is not None:
            self.ball_id = id
            self.window.createAsset(
                self.ball_id, ball_radius * 2, ball_radius * 2, imgPath=ball_img_path)
            id += 1

After:

        # Setting up ball
        self.ball_id = id
        self.window.createAsset(
            self.ball_id, ball_radius * 2, ball_radius * 2, imgPath=rospy.get_param("~BALL_IMG_PATH"))
        id += 1

or

        # Setting up ball
        ball_img_path = rospy.get_param("~BALL_IMG_PATH")
        self.ball_id = id
        self.window.createAsset(
            self.ball_id, ball_radius * 2, ball_radius * 2, imgPath=ball_img_path)
        id += 1