[releeph] Conduit failure with bad IDs

Summary:
Instead of returning a blank result it throws exceptions. Fix this up a
little so we get some consistency with differential

Test Plan:
Loaded a bad phid for releeph, returns empty list.
Try a good phid and get 2 releeph merges.

Reviewers: epriestley, elenaperezrioja, dschleimer, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7302