Start process with correlation key rest api should return 400 http status if correlation key already exist.

Customer is using the kie-server REST API to create a instance process with correlationKey using the following URL:

POST  /server/containers/{containerId}/processes/{processId}/instances/correlation/{correlationKey}

When they try to create a process instance with a duplicated correlationkey the API returns a http 500 error code. Ideally it should return a 400 error.