Display details about validation errors

As a user of the Graylog REST API, I would like to see why the validation of a request failed.

Graylog is using [bean validation](http://beanvalidation.org/) and especially [Jersey Bean Validation](https://jersey.java.net/documentation/2.21/bean-validation.html) to validate DTOs in several places.

If an HTTP request to the Graylog REST API contains an invalid payload, the default ExceptionMapper of the Jersey Bean Validation feature generates an empty response with HTTP status 400.

Jersey Bean Validation supports printing the original `ConstraintViolationExcpetion` with a simple property (https://jersey.java.net/documentation/2.21/bean-validation.html#d0e13826) but the format doesn't match the other error responses, which is why Graylog should implement its own exception mapper for `ConstraintViolationException`s.