[JSB] cleanup the activation error message

[saikishor]

Fixes #1580

I also cleaned up the other message, as this will never happen, and it would fail much earlier to this activate callback

https://github.com/ros-controls/ros2_control/blob/cc3491c9935b908cba2135fa12f629c7e5063e58/controller_manager/src/controller_manager.cpp#L1916-L1942

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.09%. Comparing base (210358b) to head (88a9b91).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1584      +/-   ##
==========================================
- Coverage   85.12%   85.09%   -0.03%     
==========================================
  Files         124      124              
  Lines       11708    11707       -1     
  Branches      997      999       +2     
==========================================
- Hits         9966     9962       -4     
- Misses       1430     1431       +1     
- Partials      312      314       +2     

Flag	Coverage Δ
unittests	85.09% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
..._state_broadcaster/src/joint_state_broadcaster.cpp	88.96% <ø> (-0.79%)	⬇️

... and 3 files with indirect coverage changes

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[christophfroehlich]

LGTM