Fixed crash when ScribbleView failed to save.

Previously, if an operation failed, we set the result to "null".
However, this was a mistake. Setting the result at all assumes success.
Instead, we need to set an exception so the ListenableFuture knows that
the operation failed.

src/org/thoughtcrime/securesms/scribbles/ScribbleActivity.java

@@ -14,6 +14,7 @@ import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 import android.util.Log;
 import android.view.View;
+import android.widget.Toast;
 
 import org.thoughtcrime.securesms.PassphraseRequiredActionBarActivity;
 import org.thoughtcrime.securesms.R;
@@ -242,6 +243,8 @@ public class ScribbleActivity extends PassphraseRequiredActionBarActivity implem
       @Override
       public void onFailure(ExecutionException e) {
         Log.w(TAG, e);
+        Toast.makeText(ScribbleActivity.this, R.string.ScribbleActivity_save_failure, Toast.LENGTH_SHORT).show();
+        finish();
       }
     });
   }