Return an error from project index tool when embedding query fails (#11264)

Previously, a failure to embed the search query (due to a rate limit
error) would appear the same as if there were no results.

* Avoid repeatedly embedding the search query for each worktree
* Unify tasks for searching all worktree

Release Notes:

- N/A

crates/semantic_index/examples/index.rs

@@ -92,10 +92,11 @@ fn main() {
                 .update(|cx| {
                     let project_index = project_index.read(cx);
                     let query = "converting an anchor to a point";
-                    project_index.search(query, 4, cx)
+                    project_index.search(query.into(), 4, cx)
                 })
                 .unwrap()
-                .await;
+                .await
+                .unwrap();
 
             for search_result in results {
                 let path = search_result.path.clone();