Need to press acknowledge again after startup, else the controller is in holding mode

[jschleicher]

* We need to press acknowledge again after startup, else the controller is in holding mode.

Yes, that's the current behaviour - just tested with the PRBT. But from a user's perspective an incorrect assumption. Why shouldn't I be able to move the robot, if acknowledge is pressed during launch? I don't like to change the acceptance test but rather fix the implementation. Is this a regression?

Originally posted by @jschleicher in https://github.com/PilzDE/pilz_robots/pull/447#issuecomment-702689688

[jschleicher]

During startup, the console shows an error

[ INFO] [1601639295.430965133]: Recovering...
[ INFO] [1601639295.431048654]: Recovering successful
[ERROR] [1601639295.433443366]: Service: /prbt/manipulator_joint_trajectory_controller/unhold failed with error message:
Could not switch to unhold mode (default mode)