payload_deliverer: Fix Gripper unitialized message spamming

- When the gripper was uninitialized, the error message popped up
everytime the module had 'any' vehicle command received. But we should
be spamming only when we do actually get 'DO_GRIPPER' command received,
this commit adds that feature
- Also converted error messages for CLI when gripper test functinality
fails, so that user would get a feedback (originally, the PX4_DEBUG was
of course not outputting everything)

src/modules/payload_deliverer/payload_deliverer.h

@@ -101,6 +101,16 @@ private:
 	 */
 	bool initialize_gripper();
 
+	/**
+	 * @brief Update gipper instance's state and send vehicle command ack
+	 *
+	 * This updates the gripper instance to check if the gripper has reached the desired state.
+	 * And if so, it sends a vehicle command ack to the navigator.
+	 *
+	 * If the gripper instance isn't valid (i.e. not initialized), it doesn't do anything
+	 */
+	void gripper_update(const hrt_abstime &now);
+
 	/**
 	 * @brief Commands the payload delivery mechanism based on the received vehicle command
 	 *