feat(exceptions): add DirectoryNotEmptyError

Added an OSError subclass for indicating that the directory is not
empty. This exception is explicitly handled by the CLI now.

src/jcloud_docsgen/cli/cli.py

@@ -20,6 +20,7 @@ import argparse
 import pathlib
 import sys
 
+from ..exceptions import DirectoryNotEmptyError
 from .languages.python import VALID_SECTIONS as VALID_PYTHON_SECTIONS
 
 __all__ = [
@@ -157,6 +158,8 @@ def main(argv=None) -> int:
         return error(f'{exc.filename}: not a directory')
     except IsADirectoryError as exc:
         return error(f'{exc.filename}: is a directory')
+    except DirectoryNotEmptyError as exc:
+        return error(f'{exc.filename}: directory not empty')
     except FileExistsError as exc:
         return error(f'{exc.filename}: file already exists')
     except PermissionError as exc: