text: Title: Use of custom error handler is obscure
  Body: Not sure if this is intended or otherwise, but just posting my experience -
  
  I am using flask_restplus with payload validation:
  ```
  class resource(Resource):
      @ns.expect(payload_model, validate=True)
  ```
  However, I want to change the default output validation json output. From the restplus source code, there seem to be no easy way to overwrite the default output validation message other than changing the source code directly at the `validate` method of the `BaseModel` class.
  
  
  I found another method which is to use the `errorhandler` provided by flask_restplus to catch the `BadRequest` raised from the `validate` function. However it seems that the error handler always uses the `data` attribute from the raised error regardless of the result I return in the handler function. The only way I got it to work is to delete the data attribute of the raised attribute.
  
  ```
  @ns.errorhandler(BadRequest)
  def handle_generic_exception(error):
      data = {'custom_data': ''}
      delattr(error, 'data')
      return data, 400
  ```