Handle errors more granularly when polling for specific packages

[tom--pollard]

With #99 landing, the ability to provide a specific set of packages for polling was added to the pypi feed, with the framework for this to be extended to other similar feeds.

This brings with it the caveat that if when polling a feed for any of the set of packages fails (e.g, network error/timeout), then the whole set of package data from that specific polling session is lost. This is due to the assumption from the generic polling of the 'firehose' that only one roundtrip will be performed, as oppose to the N that may be done if watching a given set of endpoints.

• https://github.com/ossf/package-feeds/blob/main/feeds/pypi/pypi.go#L149

• https://github.com/ossf/package-feeds/blob/main/feeds/scheduler/scheduler.go#L49

While it's right to report the error/errors, if a partial set of packages were successfully polled then it's reasonable to continue processing them and could be a candidate for the event framework.