Make the protocol version more descriptive - helps to understand when / how px4io detect fails.

src/drivers/px4io/px4io.cpp

@@ -580,6 +580,12 @@ PX4IO::init()
 	/* get some parameters */
 	unsigned protocol = io_reg_get(PX4IO_PAGE_CONFIG, PX4IO_P_CONFIG_PROTOCOL_VERSION);
 
+	if (protocol == _io_reg_get_error) {
+		log("failed to communicate with IO");
+		mavlink_log_emergency(_mavlink_fd, "[IO] failed to communicate with IO, abort.");
+		return -1;
+	}
+
 	if (protocol != PX4IO_PROTOCOL_VERSION) {
 		log("protocol/firmware mismatch");
 		mavlink_log_emergency(_mavlink_fd, "[IO] protocol/firmware mismatch, abort.");