Add a hint for the undefined factor nodes

[bvdmitri]

Fixes https://github.com/orgs/ReactiveBayes/discussions/376

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.56%. Comparing base (05f0394) to head (c68f74c).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #431      +/-   ##
==========================================
+ Coverage   71.93%   72.56%   +0.62%     
==========================================
  Files         190      190              
  Lines        5466     5493      +27     
==========================================
+ Hits         3932     3986      +54     
+ Misses       1534     1507      -27     

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