Improve exception raised when an event listener fails to load

Summary: See IRC. The failing class might not be the listener itself.

Test Plan:
  $ arc list
  ERROR: Failed to load event listener 'depr': Failed to load class or interface 'depr': the class or interface 'depr' is not defined in the library map for any loaded phutil library. If this symbol was recently added or moved, your library map may be out of date. You can rebuild the map by running 'arc liberate'. For more information, see: http://www.phabricator.com/docs/phabricator/article/libphutil_Libraries_User_Guide.html

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7616