Fix connection failure logic (#604)

* refactor: Adds extra logging in proposal extraction

* style: Initial cache now a set rather than empty list

* fix: Fix cache collection failure logic

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Co-authored-by: Alan Christie <alan.christie@matildapeak.com>

api/security.py

@@ -253,7 +253,6 @@ def _get_proposals_for_user_from_ispyb(self, user):
                 self._get_proposals_from_connector(user, conn)
             else:
                 logger.warning("Failed to get a connector for '%s'", user.username)
-                self._mark_cache_collection_failure(user)
         else:
             PrometheusMetrics.new_proposal_cache_hit()