Add an error msg if empty message is received

[christophfroehlich]

There existed one branch where a goal is rejected but no error message is given.

[saikishor]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.72%. Comparing base (d32665a) to head (4013b49).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1424   +/-   ##
=======================================
  Coverage   83.71%   83.72%           
=======================================
  Files         122      122           
  Lines       11050    11052    +2     
  Branches      940      940           
=======================================
+ Hits         9251     9253    +2     
  Misses       1489     1489           
  Partials      310      310           

Flag	Coverage Δ
unittests	83.72% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
...ory_controller/src/joint_trajectory_controller.cpp	83.95% <100.00%> (+0.04%)	⬆️