change Err from Debug to Display

as reported by a user on Discord, the output of the Debug on error is a useless: 'Any { .. }'
switch to `Display` via to_string

src/indexer/index_writer.rs

@@ -620,7 +620,7 @@ impl IndexWriter {
         for worker_handle in former_workers_join_handle {
             let indexing_worker_result = worker_handle
                 .join()
-                .map_err(|e| TantivyError::ErrorInThread(format!("{e:?}")))?;
+                .map_err(|e| TantivyError::ErrorInThread(e.to_string()))?;
             indexing_worker_result?;
             self.add_indexing_worker()?;
         }