NEW Provide clear UX validation errors when violating unique index #3647

Job	Run time
CI / Context	1s
CI / Generate matrix	22s
CI / 8.1 prf-low mysql57 phpunit framework-core	5m 27s
CI / 8.2 mariadb phpunit framework-core	5m 17s
CI / Check governance	0s
CI / 8.3 mysql80 phpunit framework-core	6m 38s
CI / 8.3 mysql84 phpunit framework-core	7m 1s
CI / 8.1 prf-low mysql57 phpunit framework-orm	11m 36s
CI / 8.2 mariadb phpunit framework-orm	11m 13s
CI / 8.3 mysql80 phpunit framework-orm	12m 43s
CI / 8.3 mysql84 phpunit framework-orm	12m 35s
CI / 8.1 prf-low mysql57 phpunit cms	1m 29s
CI / 8.2 mariadb phpunit cms	1m 24s
CI / 8.3 mysql80 phpunit cms	1m 47s
CI / 8.3 mysql84 phpunit cms	1m 32s
CI / 8.1 mysql57 phplinting	2m 44s
CI / Dispatch tag patch release	0s
	1h 21m 49s

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