add loadFailedBuilder rather than loadFailedChild

[Aoi-hosizora]

Version: photo_view: ^0.9.2

Is your feature request related to a problem? Please describe.

I use PhotoViewGallery with my customer ImageProvider.
I want to show the error message generated by (customer or not) ImageProvider. But I find there is only a loadFailedChild property. I hope it can wrap by a function.

Describe the solution you'd like

In photo_view.dart, use _loadFailedError rather then _loadFailed:

// 1.
Future<ImageInfo> _getImage() {
  // xxx
  }, onChunk: (event) {
    if (mounted) {
      setState(() => _imageChunkEvent = event);
    }
  }, onError: (exception, stackTrace) {
    if (!mounted) {
      return;
    }
    setState(() {
      // _loadFailed = true;
      _loadFailedError = exception ?? 'unknown error'; // <<<
   });
    /* FlutterError.reportError(
      FlutterErrorDetails(exception: exception, stack: stackTrace),
    ); */
  });
  stream.addListener(listener);
  completer.future.then((_) {
    stream.removeListener(listener);
  });
  return completer.future;
}

// 2.
@override
Widget build(BuildContext context) {
  return _loadFailedError != null
      ? _buildLoadFailed() // <<<
      : LayoutBuilder(
          builder: (
            BuildContext context,
            BoxConstraints constraints,
          ) {
            return widget.child == null
                ? _buildImage(context, constraints)
                : _buildCustomChild(context, constraints);
          },
        );
}

// 3.
Widget _buildLoadFailed() {
  return widget.loadFailedBuilder(_loadFailedError) ?? PhotoViewDefaultError(); // <<<
}

[renancaraujo]

Should be fixed by #320