Don't crash if a wrong config was detected (backport #324)

[mergify]

As reported with #323, the plugin crashes if there is a wrong config. On humble, the controller_manager tries to access unavailable state interfaces (I'll investigate more).

But also on rolling it crashes, despite the URDF parsers throws an error if a joint does not exists. The configure() method returns but PreUpdate, PostUpdate and Deconstructor are accessing the this->dataPtr->controller_manager_

gz_ros2_control/gz_ros2_control/src/gz_ros2_control_plugin.cpp

Lines 394 to 397 in 4503507

	RCLCPP_ERROR_STREAM(
	this->dataPtr->node_->get_logger(),
	"Error parsing URDF in gz_ros2_control plugin, plugin not active : " << ex.what());
	return;

Is there maybe another way to deactivate the plugin properly and not even call the Update methods?

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This is an automatic backport of pull request #324 done by Mergify.