Writing a blend file to a path that references a directory wasn't
being handled correctly.
When the save "versions" was greater than zero the directory would
be renamed (adding a 1), otherwise there would be a
"Version backup failed" error.
Resolve by exiting with an error in the unlikely event the user saves
over a directory.
Part of a fix for #134101.
Ref !134384
f2a1c8d77c