FIX: ensure there is only 1 version of DuplicateHandler
FIX: ensure there is only 1 version of DuplicateHandler

 - Let the ground truth be in handler_base
 - use the exception class stashed on the BaseRegistryRO object to
   allow customization by sub-classes and so a registry instance
   bring's its exception classes with it

closes #334

ericdill/databroker,ericdill/databroker