Error for failed constraint validation

[lucasfelber]

It would be nice to have a dedicated error for failure when validating constraints.
Right now it is an unexpected message:

unexpected response for PULL: Ok(Failure(Failure { metadata: BoltMap { value: {BoltString { value: "message" }: String(BoltString { value: "Node(2) already exists with label Label and property property = 'prop'" }), BoltString { value: "code" }: String(BoltString { value: "Neo.ClientError.Schema.ConstraintValidationFailed" })} } }))