Add ability to get an optional EID from a lookup ref

The situation comes up from time to time where I have some sort of lookup ref, such as an email address for example, but I do not know for sure if a user entity with that email address exists yet in the DB. Currently, if I try to grab the associated entity for the lookup ref, it will throw an exception. In the cases where I know that my lookup ref might not turn anything up, I'd like to be able to ask for an Option instead.