Use custom exception instead of runtime NullPointerException

[consp1racy]

NPE extends RuntimeException and the IDE does not advice catching it even if a method declares it throws NullPointerException.

Seen in BlurLayout.getDownscaledBitmapForView(...)

Use something like public class NoScreenException extends Exception {...}.

[dwillmc]

I reused the NullPointerException because there were cases where the screenView.draw(...) method would throw it, so I was handling that anyway. Thanks for the issue, I'll address this soon and add a custom exception type.

[dwillmc]

Now catching the NullPointerException thrown internally to the Android APIs, and a custom BlurKitException.