RobotLink: check if loading mesh failed before creating entity V2

[simonschmeisser]

alternative to #1596 which also fixes #1589

I had to split the RobotLink constructor in order to be able to connect to the signal before the mesh loading happens

now unfortunately MoveIt's RobotStateDisplay does not extend upon RobotModelDisplay so that will need to be extended in a follow up

[rhaschke]

I thought a little more about other alternatives to implement this.
My first idea was to use exceptions, but they would leave the loop in RobotLink (and Robot) creating all geometries (and links).
Also, the current approach only shows the last error message.
What about storing all failures in a string in RobotLink and provide a method to collect (i.e. concatenate) all these failures with a new function in rviz::Robot? This way, a display could show all of them once after instantiation of the robot.
Sorry for coming up with this idea so late.

[simonschmeisser]

This actually shows the last error per link, so yes, if your visual and collision meshes are both not found your link will have only one error but if you have multiple links with broken meshes then they will all be displayed as in error. And you can only fix one error at a time, right? (Our second use case besides not crashing is a urdf editor with preview and it would be great to have more feedback there. Currently it just does not display the mesh if you have a typo.)

Your approach with a list of problem strings that might be empty also sounds like a good idea however, I'll think about it

[simonschmeisser]

Actually the errors got overridden by successful tf updates so they now have separate sections

[simonschmeisser]

Wouldn't adding such a string break ABI compatibility? What do you think about the current state of this PR?