refactor(exceptions): define public module exports

Added an __all__ variable to the exceptions module.

src/jcloud_docsgen/exceptions.py

@@ -14,6 +14,20 @@
 
 import errno
 
+__all__ = [
+    'NamespaceError',
+    'InvalidNamespaceError',
+    'NamespaceNotFoundError',
+    'NamespaceExistsError',
+    'PythonIdentifierError',
+    'InvalidPythonIdentifierError',
+    'PythonAnnotationError',
+    'InvalidPythonAnnotationError',
+    'PythonDefinitionError',
+    'PythonArgumentStructureError',
+    'DirectoryNotEmptyError',
+]
+
 
 class NamespaceError(ValueError):
     """