ComputeIK: report failures the modern way

[rhaschke]

Don't use command-line warnings, but spawn failure solutions. This fixes #650 (comment):

Failing stage(s):
compute ik (0/4): ik frame unknown in robot: 'foo'

Additionally, to correctly report ComputeIK's failure comments, I had to adapt explainFailure().

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 21.27660% with 37 lines in your changes missing coverage. Please review.

Project coverage is 58.97%. Comparing base (8318546) to head (acb31e2).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/src/stages/compute_ik.cpp	0.00%	12 Missing ⚠️
core/src/stages/current_state.cpp	0.00%	7 Missing ⚠️
core/src/stages/fixed_cartesian_poses.cpp	0.00%	6 Missing ⚠️
core/src/stages/generate_pose.cpp	0.00%	6 Missing ⚠️
core/src/stages/generate_random_pose.cpp	0.00%	6 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #651      +/-   ##
==========================================
- Coverage   59.21%   58.97%   -0.24%     
==========================================
  Files          97       97              
  Lines        8976     9001      +25     
==========================================
- Hits         5314     5307       -7     
- Misses       3662     3694      +32     

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[DaniGarciaLopez]

Thanks for this improvement @rhaschke! There is one missing check in generate_pose.cpp here, otherwise LGTM.