[DoctrineBridge] add missing error code

Author: xabbuh

Tests/Validator/Constraints/UniqueEntityValidatorTest.php

@@ -448,6 +448,7 @@ public function testValidateUniquenessNotToStringEntityWithAssociatedEntity()
             ->atPath('property.path.single')
             ->setParameter('{{ value }}', $expectedValue)
             ->setInvalidValue($expectedValue)
+            ->setCode(UniqueEntity::NOT_UNIQUE_ERROR)
             ->assertRaised();
     }