This PR contains a series of minor changes to address issues that I happened to run into:

 - Pretty-print errors triggered when handling events that are pending because of having locked event handling. Previously these were just dumped to the console.
 - Add more documentation for `debugPaintPadding`.
 - Add documentation to `Tween` saying how to implement it.
 - Slight formatting changes in the scrollbar code to align some expressions.
 - Since we convert ScrollMetricsNotifications to ScrollNotifications in various places, provide an explicit API to do this. This will make the behaviour consistent throughout, and makes the code easier to understand. Added a test for this.
 - Clarifications to some of the BindingBase and SchedulerBinding documentation.
 - Clarified some documentation in `flutter_test`'s `Finder` class.