Raise error in generate_strongly_connected_components instead of asserting

[Robbybp]

Fixes #3297

More useful error message.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.49%. Comparing base (f5e6f32) to head (fe79414).
Report is 860 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3305      +/-   ##
==========================================
- Coverage   88.50%   88.49%   -0.01%     
==========================================
  Files         868      868              
  Lines       98427    98430       +3     
==========================================
- Hits        87112    87107       -5     
- Misses      11315    11323       +8     

Flag	Coverage Δ
linux	86.30% <100.00%> (+<0.01%)	⬆️
osx	75.61% <100.00%> (-0.01%)	⬇️
other	86.50% <100.00%> (+<0.01%)	⬆️
win	83.81% <100.00%> (+<0.01%)	⬆️

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