Move index swap error handling from meilisearch-http to index-scheduler

And make index_not_found error asynchronous, since we can't know
whether the index will exist by the time the index swap task is
processed.

Improve the index-swap test to verify that future tasks are not swapped
and to test the new error messages that were introduced.

meilisearch-http/tests/tasks/mod.rs

@@ -705,7 +705,7 @@ async fn test_summarized_index_swap() {
         ]
       },
       "error": {
-        "message": "Index `doggos` not found.",
+        "message": "Indexes `cattos`, `doggos` not found.",
         "code": "index_not_found",
         "type": "invalid_request",
         "link": "https://docs.meilisearch.com/errors#index_not_found"