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.

res/values/strings.xml

@@ -584,6 +584,9 @@
     <string name="RingtonePreference_add_ringtone_text">Add ringtone</string>
     <string name="RingtonePreference_unable_to_add_ringtone">Unable to add custom ringtone</string>
 
+    <!-- ScribbleActivity -->
+    <string name="ScribbleActivity_save_failure">Failed to save image changes</string>
+
     <!-- Search -->
     <string name="SearchFragment_begin_searching">Begin typing to search for conversations, messages, and contacts.</string>
     <string name="SearchFragment_no_results">No results found for \'%s\'</string>

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();
       }
     });
   }

src/org/thoughtcrime/securesms/scribbles/widget/ScribbleView.java

@@ -98,7 +98,7 @@ public class ScribbleView extends FrameLayout {
     final boolean                isLowMemory = Util.isLowMemory(context);
 
     if (imageUri == null) {
-      future.set(null);
+      future.setException(new IllegalStateException("No image URI."));
       return future;
     }
 
@@ -126,7 +126,7 @@ public class ScribbleView extends FrameLayout {
 
                    @Override
                    public void onLoadFailed(@Nullable Drawable errorDrawable) {
-                     future.set(null);
+                     future.setException(new Throwable("Failed to load image."));
                    }
                });