[RM-6295] throw execption in resulstore getter

Created on 2021-01-29 09:03:02 by Didier Vibert. % Done: 100

For instance in @COperatorResultStore::GetGlobalResult@ the getter is returning a pointer to an empty result (not a nullptr) when the result asked for is not in the resultstore.

TODO:

• throw an exceptions in all the getters
• add a method to query the existence of a result in the store