Add 'event' for per package poll/publish errors

[tom--pollard]

With #123 & #132 error handling has been made a lot more granular, in certain cases at a per package level. In cases where a poll or publish process fails and is not a 'hard' error, it could be beneficial to create a new 'lossy' type of event. https://github.com/ossf/package-feeds/blob/main/events/README.md