Proper Error Handling

[prokopyl]

This PR removes many (many (many)) cases of panics and unwrap()s within baseview, and replaces them with proper error handling:

• Add a new generic, opaque Error for all window-related operations.
• Add a new HandlerError that can coalesce any error type, to be returned in WindowHandler methods.
• WindowHandler::on_frame can now return errors, which will properly destroy the window if they do.
• WindowHandler::resized can now return errors, which will revert the resize operation if they do.
• create_window now returns a Result, and the WindowHandler builder it takes is now also fallible.
• Many methods on WindowContext now return an Error when they fail, instead of panicking.