Improve status/error reporting of point queue service (and related functionality)

[gavanderhoorn]

Context: #230 & #335.

QueueTrajPoint returns a result_code, with one of the values defined in QueueResultEnum.msg.

As evidenced by the two references discussions however, these result codes are insufficient when trying to diagnose failures that might occur when enqueuing (new) trajectory points.

We should improve status and error reporting of that service, such that usage of the debug logger should not be required in order to understand what's going on.