Add error reporting in _addKnownFiles for missing imported/exported libraries

[jcollins-g]

It is possible for a package to show no errors via dart analyze but have an incomplete import/export graph under some rare circumstances. Add error reporting in _addKnownFiles to surface this situation to the user because it can cause crashes in dartdoc, later.

[jcollins-g]

There is one case I know of where the import/export graph can be invalid, and that's because the way that sky_engine is structured for Flutter leads to this case because of how Flutter extends the Dart SDK. That can be worked around though as that's a known safe case.