Fix EZP-20606 REST better error message

This PR fixes issue https://jira.ez.no/browse/EZP-20606

REST specification is updated for Create ContentType resource, defining ```Bad Request 400``` if field definitions do not validate.

Additionaly phpdoc for ```ContentTypeService::createContentType()``` interface and implementations is updated, adding  ```ContentTypeFieldDefinitionValidationException``` if field definitions do not validate.