Log and continue when trying to register an invalid event listener

Summary: We currently die if an event listener throws when registering (e.g., because it is misconfigured), but this prevents you from running bin/config to fix it, which is a bit silly.

Test Plan: Created this revision with an invalid listener in config.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6761