tags: REST API and testing

* refactors errors class for tags

* refactors ther error handler in restful api for tags

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

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

restful: addition of RestValidator class

* Adds support to check if a string is
  in UTC ISO format.

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