JointPositionController: Improve misleading error message

[chapulina]

🦟 Bug fix

Summary

If another plugin adds the JointPosition component to the joint entity before the position controller does, it was printing this misleading message:

[Err] [JointPositionController.cc:256] [JointPositionController]: Detected an invalid <joint_index> parameter. The index specified is [0] but the joint only has [0] index[es]. This controller will be ignored

However, what's happening is that we need to wait one iteration for the physics system to populate the component.

The fix here means that the plugin will silently skip updates for joints that have zero axes, such as ball joints, though.

I also made sure that if the plugin is instantiated multiple times for multiple joints, the error message is printed for all of them.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #1098 (d4bab48) into ign-gazebo3 (d9195c4) will increase coverage by 0.28%.
The diff coverage is 78.24%.

@@               Coverage Diff               @@
##           ign-gazebo3    #1098      +/-   ##
===============================================
+ Coverage        77.56%   77.85%   +0.28%     
===============================================
  Files              208      221      +13     
  Lines            11385    12723    +1338     
===============================================
+ Hits              8831     9905    +1074     
- Misses            2554     2818     +264     

Impacted Files	Coverage Δ
include/ignition/gazebo/EntityComponentManager.hh	100.00% <ø> (ø)
include/ignition/gazebo/Link.hh	100.00% <ø> (ø)
include/ignition/gazebo/Server.hh	100.00% <ø> (ø)
...nclude/ignition/gazebo/components/ContactSensor.hh	100.00% <ø> (ø)
include/ignition/gazebo/components/Factory.hh	96.05% <ø> (ø)
...nclude/ignition/gazebo/components/LogicalCamera.hh	100.00% <ø> (ø)
include/ignition/gazebo/components/Sensor.hh	100.00% <ø> (ø)
...nclude/ignition/gazebo/components/Serialization.hh	90.32% <ø> (ø)
include/ignition/gazebo/gui/GuiSystem.hh	0.00% <0.00%> (ø)
.../plugins/component_inspector/ComponentInspector.cc	8.58% <0.00%> (-0.90%)	⬇️
... and 111 more

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[arjo129]

Thanks! The error looks much clearer now! I've got a minor knit, it doesn't really matter as there should be a limited number of joints.

[j-rivero]

first quick look at the code, looks good, one important question only