Improve feedback on planning errors

When a kinematic (and to some degree also cartesian) path plan fails to find a solution, the cause of error is really obscure and it's not surfaced to the user. Sometimes MoveIt! does have the information (eg. start state or goal state are invalid), and we should show it more clearly.

Start by reproducing a failed kinematic path plan and inspect how MoveIt! reports invalid start or goal states. The work is complete when available planning-error causes are surfaced clearly to users for kinematic and, where possible, Cartesian planning failures.

Written by the indexing model from the issue text.