show error message if shared URL is not understood

Avoid the dreaded blank white screen!
make internal vfs: URLs "hierarchical" to that they are properly parsed

Since the internal vfs: URLs are meant to represent a path, they need to be
proper "hierarchical URLs" that include an "authority".  The authority is
the part between :// and /.  You can have a blank authority, like with
file:/// URLs.  That's what's happening here.