tags: REST API and testing

* Removes properties, setters and getters, from
  the errors class.

* Fixes delete function for the tags list
  resource.

* Adds more tests

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>
restful: fix for testing accesstoken

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