Define ErrRelationNotFound error in searchsvc package. Return
ErrRelationNotFound from RelationSearcher.SearchRelation method if search
result is empty.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
If object service produces new request, the should contain
session token. This is the only way for node to grant access
for a private container.
Signed-off-by: Alex Vanin <alexey@nspcc.ru>