restful: addition of RestValidator

* Adds a custom validator for the messages
  restful api.

* Adds a dictionary of validation errors
  for the RestValidator.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>
messages: addition of REST API

* Adds an errors class for the messages
  restful api.

* Adds a MessageValidationError class to the errors.

* Adds an error handler for the messages
  restful api.

* Tests the messages restful api.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>