make find_dandi_files raise FileNotFoundError exception if path is invalid

ATM it just returns empty list and that is it:

```
(git)lena:~/proj/dandi/dandi-cli-master[master]git
$> python3 -c 'import dandi.files as df; from pprint import pprint; pprint(list((hex(id(x)), x) for x in df.find_dandi_files("/home/yoh/datalad/openneuro/ds00001")))'
[]

$> ls -l /home/yoh/datalad/openneuro/ds00001
ls: cannot access '/home/yoh/datalad/openneuro/ds00001': No such file or directory
```

initially observed while testing #1076 and there behavior is identical. Needs analysis on what this would cause in terms of UI/UX.