actors: `message-ref' now errors if no dflt provided and key not found

actors: `message-ref' now errors if no dflt provided and key not found

Thanks to David Thompson for guidance on the best way to construct
%nothing-provided.

* 8sync/systems/actors.scm (message-ref): Adjust to error if no dflt
  provided and key isn't found.
  (%nothing-provided): New unique, immutable value for checking
  if dflt provided to message-ref.