NEW Provide clear UX validation errors when violating unique index #3660

Job	Run time
CI / Generate matrix	12s
CI / Context	1s
CI / 8.1 prf-low mysql57 phpunit framework-core	6m 6s
CI / 8.2 mariadb phpunit framework-core	5m 14s
CI / Check governance	0s
CI / 8.3 mysql80 phpunit framework-core	8m 47s
CI / 8.3 mysql84 phpunit framework-core	7m 0s
CI / 8.1 prf-low mysql57 phpunit framework-orm	12m 0s
CI / 8.2 mariadb phpunit framework-orm	11m 20s
CI / 8.3 mysql80 phpunit framework-orm	13m 27s
CI / 8.3 mysql84 phpunit framework-orm	13m 8s
CI / 8.1 prf-low mysql57 phpunit cms	1m 58s
CI / 8.2 mariadb phpunit cms	1m 50s
CI / 8.3 mysql80 phpunit cms	1m 25s
CI / 8.3 mysql84 phpunit cms	1m 30s
CI / 8.1 mysql57 phplinting	2m 44s
CI / Dispatch tag patch release	0s
	1h 26m 42s

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