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/exceptions.py

@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import errno
+
 
 class NamespaceError(ValueError):
     """
@@ -101,3 +103,12 @@ class PythonDefinitionError(ValueError):
 
 
 class PythonArgumentStructureError(PythonDefinitionError): ...
+
+
+class DirectoryNotEmptyError(OSError):
+    """
+    Directory is not empty.
+    """
+
+    def __init__(self, filename: str | None = None) -> None:
+        super().__init__(errno.ENOTEMPTY, 'Directory not empty', filename)