Fix SolumBase _raise_not_found() 'glued' to the API due to __resource__

* Added ObjectNotFound and ObjectNotUnique exception for database entities
* Renamed previous NotFound exception in ResourceNotFound
* _raise_not_found() in SolumBase (sqlalchemy base class) is not anymore
glued to the API: before this change, _raise_not_found
couldn't be used  with sqlalchemy objects that are not associated to
any API resource, Indeed these objects don't have a __resource__
defined and _raise_not_found needed this field.
* Applied the same behavior to ResourceExists and then also created
ObjectNotUnique specific exception to database entities.

Change-Id: I6ff7d90562e4f03ecaf7cea52d03cc46caee51c3