Catch errors raised by edition service observers

This is a quick fix to address in particular the issue that scheduled
publishing could fail if any of the observers fail. We had an problem
where the email service failed and the AuthorNotifier failed, causing
scheduled publishing to fail.

This needs to be revisited to provide a proper fix, in particular:

* the main service object should run in a transaction
* the failure of one observer should not prevent other observers from
  firing

See https://www.pivotaltracker.com/story/show/64036694
Merge pull request #1221 from alphagov/rescue-edition-service-errors

Catch errors raised by edition service observers

[Fixes https://www.pivotaltracker.com/story/show/63956368]