I13 display plot warnings

[hillalex]

• don't attempt to render facets for null factor levels
• display facet errors in place of empty plots
• display trace warnings below facets

Closes #13

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.61%. Comparing base (8c76677) to head (b5ed723).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
+ Coverage   97.86%   98.61%   +0.74%     
==========================================
  Files          21       23       +2     
  Lines         468      504      +36     
  Branches       89      103      +14     
==========================================
+ Hits          458      497      +39     
+ Misses         10        7       -3     

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