Add a `summary` property to `ValidationError` exceptions #501

Job	Run time
Misc tests (checks)	1m 12s
Unit tests (py38-unittest)	1m 38s
Integration tests (py38-integration)	3m 57s
Unit tests (py39-unittest)	1m 33s
Integration tests (py39-integration)	3m 59s
Unit tests (py310-unittest)	1m 33s
Integration tests (py310-integration)	4m 8s
Unit tests (py311-unittest)	1m 32s
Integration tests (py311-integration)	3m 58s
Unit tests (py312-unittest)	1m 28s
Integration tests (py312-integration)	4m 5s
Build distribution 📦	10s
Publish to PyPI 🚀	0s
Create a GitHub Release 📢	0s
	29m 13s

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