Fix bug in `bin/omero admin diagnostics`

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/11665

To test this PR, call `bin/omero admin diagnostics` and make sure the command outputs either `Exists: True` or `Exists: false` for the OMERO data directory and the OMERO temp directory.

```
$ bin/omero admin diagnostics
…
OMERO data dir: '/OMERO'	Exists? True	Is writable? True
OMERO temp dir: '/Users/sebastien/omero/tmp'	Exists? True	Is writable? True   (Size: 3358)
```

----

--no-rebase