moveit_cpp: Cover another failure path in planning pipeline loading

[AndyZe]

Add a more informative error message, and cover another potential failure condition.

[codecov]

Codecov Report

Merging #2336 into master will decrease coverage by 0.01%.
The diff coverage is 33.34%.

@@            Coverage Diff             @@
##           master    #2336      +/-   ##
==========================================
- Coverage   56.73%   56.73%   -0.00%     
==========================================
  Files         283      283              
  Lines       25455    25457       +2     
==========================================
+ Hits        14440    14441       +1     
- Misses      11015    11016       +1     

Impacted Files	Coverage Δ
...s/planning_interface/moveit_cpp/src/moveit_cpp.cpp	64.17% <33.34%> (-0.24%)	⬇️
...ma_kinematics_plugin/src/lma_kinematics_plugin.cpp	71.23% <0.00%> (-3.59%)	⬇️
...raint_samplers/src/default_constraint_samplers.cpp	82.17% <0.00%> (-0.34%)	⬇️
moveit_core/robot_state/src/robot_state.cpp	46.24% <0.00%> (+0.10%)	⬆️
...dl_kinematics_plugin/src/kdl_kinematics_plugin.cpp	76.96% <0.00%> (+0.44%)	⬆️
...nning_scene_monitor/src/planning_scene_monitor.cpp	68.97% <0.00%> (+0.45%)	⬆️
...on/pick_place/src/approach_and_translate_stage.cpp	73.39% <0.00%> (+0.81%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 89d63e8...9c6c08c. Read the comment docs.

[AndyZe]

Accidentally closed this by pushing an empty changeset

[tylerjw]

More helpful error messages are always nice. Thank you.