Show cause of error when opening drag-&-dropped files fails

Plumb the error code back from the `FILE_OPEN` command, and display errors in the existing bullet list error dialog used by DragAndDrop.
This is fine to re-enable these but this one should be renamed to `opening an image file sets the current document state to null` or something. The official `open image file` is in `MainViewFactory-test.js`.  Those tests don't check for things like `DM.getCurrentDocument().toBeFalsey()` but it does check that `MVM.getCurrentlyViewedFile()` returns the name of of the image that was opened.