add exceptions package

As per the discussion in [1] extension specific exceptions that are
not API facing would best live outside the api-def itself. This patch
suggests added an exceptions package that will allow per-component
exception definitions as sub-modules (ex: neutron_lib.exceptions.l3).
The existing exceptions from neutron_lib.exceptions.py are defined
in neutron_lib.exceptions.__init__ and don't impact existing consumers
import them.

[1] https://review.openstack.org/#/c/423289/

Change-Id: Ie0797f25b99c68bdf5cc1a58c6b906cf850699a4

neutron_lib/tests/unit/callbacks/test_callback_exceptions.py

@@ -20,7 +20,7 @@ Tests for `neutron_lib.callback.exceptions` module.
 import functools
 
 import neutron_lib.callbacks.exceptions as ex
-from neutron_lib.tests.unit import test_exceptions
+from neutron_lib.tests.unit.exceptions import test_exceptions
 
 
 class TestCallbackExceptions(test_exceptions.TestExceptions):

releasenotes/notes/add-exception-pkg-5a14389891abf358.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - A new ``neutron_lib.exceptions`` package is now available where the
+    existing definitions from the ``exceptions`` module are now in
+    ``neutron_lib.exceptions.__init__`` and subsequent per-component
+    exceptions can be created in their own exceptions sub-module.