Add try except to is_admin check. Fix #645

can you actually do this:
```python
if self.current_user:
    try:
        user = User(self.current_user)
    except Exception as e:
        # Log the error on the log table
    else:
        is_admin = user.level == 'admin'
```
The point is that I think the only think that can go wrong is the instantiation of the user.