Add error_string to FollowJointTrajectory response

[adolfo-rt]

As of now, the result of a FollowJointTrajectory action is an error_code, which is partially informative. It lets you know whether execution was successful or not, and in case of failure, a general assessment of the failure type.

Still, I'd find it useful to have some additional information for at least user feedback.

Usecase

A motion fails because goal time constraints were violated for joint foo.

Current state

The error_code of the action result message states GOAL_TOLERANCE_VIOLATED. Assuming we're using the joint_trajectory_controller from ros_controllers in a realtime deployment, no log statements are issued, as currently there is no mechanism for logging from a realtime context.

Proposal
For most cases I think that it would suffice to create a complementary error_string member containing messages like:

• In case of INVALID_JOINTS:

Command specifies a trajectory for joint 'bar', which is not part of
controller 'right_arm' .

• In case of GOAL_TOLERANCE_VIOLATED:

Goal tolerance violated for joint 'foo'. Position error: 0.03 (max 0.01).

Although we could set up a mechanism for logging these messages from a realtime context, I think it's still relevant to add this to the action spec, so that the message can be used in the context of the client, like displaying it in a GUI.

[sachinchitta]

+1 for adding an error_string member

[davetcoleman]

+1 this would be great

[adolfo-rt]

You can find a patch with a proposed doc for the new field in PR #5.

[adolfo-rt]

The resolution of this issue seems to be blocking the release into Indigo, due to package version issues, see #6.

[adolfo-rt]

Changes merged and released into indigo as control_msgs 1.3.0.