Add tests for error handler

[mrbean-bremen]

• add error handler example
• add some rudimentary documentation
• minor refactoring

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.53957% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.52%. Comparing base (e8644e2) to head (6e49f68).
⚠️ Report is 138 commits behind head on main.

Files with missing lines	Patch %	Lines
dicom_validator/validator/html_error_handler.py	70.52%	28 Missing ⚠️
dicom_validator/validator/error_handler.py	80.00%	4 Missing ⚠️
dicom_validator/spec_reader/edition_reader.py	75.00%	1 Missing ⚠️
dicom_validator/validator/iod_validator.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
- Coverage   89.82%   88.52%   -1.31%     
==========================================
  Files          15       21       +6     
  Lines        1632     2039     +407     
==========================================
+ Hits         1466     1805     +339     
- Misses        166      234      +68     

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