`dbm` errors should contain file names when appropriate, just like `OSError`.
E.g.:
```
>>> open("fooooo")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
FileNotFoundError: [Errno 2] No such file or directory: 'fooooo'
>>> import dbm.gnu
>>> dbm.gnu.open("fooooo")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
_gdbm.error: [Errno 2] No such file or directory
```
the `_gdbm.error` should report the file name `'fooooo'` just like `FileNotFoundError` does (and `exn.args` should contain the file name too!)