datastore plugin's permission checks will fail on a non english environnement

ckanext/datastore/plugin.py is using the error messages returned by postgresql to check if the permisisons are right, for example:

```python
if 'permission denied' in str(e) or 'read-only transaction' in str(e):
                        pass
```
and
```python
 if 'permission denied' not in str(e):
                        raise
```

this will fail if postgresql running in a non english localized environnement since it seems that postgresql errors are translated (verified with postesql 8.4 and 9.1 on release-v2.0)