python: do not bail out when file is not found

After a file not being found, the python interface should ideally return to the python interpreter:

```
>>> facs.query("jarl", "karl")
karl: No such file or directory
>>> 
```

Instead of exiting to the underlying shell as it does now:

```
>>> facs.query("jarl", "karl")
karl: No such file or directory
$ 
```
