rabbit_db: Fix rabbit_db_msup:update_all/2 spec

[Why]
It can return an error.

(cherry picked from commit 913bd9f)

Author: dumbbell

deps/rabbit/src/rabbit_db_msup.erl

@@ -226,9 +226,10 @@ find_mirror_in_khepri(Group, Id) ->
 %% update_all().
 %% -------------------------------------------------------------------
 
--spec update_all(Overall, Overall) -> [ChildSpec] when
+-spec update_all(Overall, Overall) -> [ChildSpec] | Error when
       Overall :: pid(),
-      ChildSpec :: supervisor2:child_spec().
+      ChildSpec :: supervisor2:child_spec(),
+      Error :: {error, any()}.
 
 update_all(Overall, OldOverall) ->
     rabbit_khepri:handle_fallback(