Introduce Exception handler for when no result needs to be returned

Also add NotNull annotation to classes that were touched

briar-android/src/org/briarproject/android/controller/handler/UiExceptionHandler.java

@@ -0,0 +1,34 @@
+package org.briarproject.android.controller.handler;
+
+import android.support.annotation.UiThread;
+
+import org.briarproject.android.DestroyableContext;
+import org.briarproject.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public abstract class UiExceptionHandler<E extends Exception>
+		implements ExceptionHandler<E> {
+
+	protected final DestroyableContext listener;
+
+	protected UiExceptionHandler(DestroyableContext listener) {
+		this.listener = listener;
+	}
+
+	@Override
+	public void onException(final E exception) {
+		listener.runOnUiThreadUnlessDestroyed(new Runnable() {
+			@Override
+			public void run() {
+				onExceptionUi(exception);
+			}
+		});
+	}
+
+	@UiThread
+	public abstract void onExceptionUi(E exception);
+
+}