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.
how about changing this param to `path` since it's not a File object? I know that's just a minor nit but I've actually gotten bitten by this by changing some code thinking that it was a file object and doing something like `file.fullPath` and getting an RTE because it was actually a path string being passed not a File object.