check if envdir is a directory

If a file path was specified as envdir directory, py-envdir tried to read env files from `'filepath/*'`.
Since glob does not care if these paths exist or not, instead of raising error,
glob just returns `[]` and no environment variables are overwritten.

On djb's original implementation, envdir raises 111 error for such case.

# djb envdir's behavior::

```
  $ /usr/bin/envdir /etc/hostname ls
  envdir: fatal: unable to switch to directory /etc/hostname: not a directory
  $ echo $?
  111
```
Changed py-envdir to bahave just like djs envdir.