Calling nwb_file.create() on the same file twice generates a mysterious error message

When running 'create()' on an existing file, generates the following error message:

```
526 
527         try:
```

--> 528             line = py3compat.cast_unicode_py2(self.raw_input_original(prompt))
    529         except ValueError:
    530             warn("\n********\nYou or a %run:ed script called sys.stdin.close()"

Ideally this should show 'File already exits' or something similar
