Update nuget & packagist feeds to support ErrNoPackagesPolled & PackagePollError

[tom--pollard]

With #123 the behaviour of feed polling became less brittle. It introduced ErrNoPackagesPolled & PackagePollError{} which were applied to the npm & pypi feeds, along with relevant tests. The behaviour was applied to all feeds where plausible but the specific error types were only deemed essential for the previously mentioned feeds as they support packages option.

Both nuget & packagist poll in a manner which can lead to per package errors. The previously mentioned error types should be extended to these feeds to provide more verbose error logging, as well as the test cases to cover 'partial' poll sets.