Attempting to mount an already mounted osxfuse mount doesn't throw an exception

Trying to mount an already mounted file system does not trigger an exception. It seems to be swallowed and the following output is produced:

```
mount_osxfusefs: mount point /private/tmp/mount is itself on a OSXFUSE volume    
```

I would have expected this to fail because the resulting file system is unusable:

```
ls /tmp/mount
ls: cannot open directory /tmp/mount: Device not configured
```
