[Log] Optimize delete error message (#6187)

## Proposed changes

When a delete error occurs, the error message is ambiguous.

```sql
mysql> DELETE FROM nebula_trade_health_trade PARTITION q3_2021 WHERE event_day = '20210706';
ERROR 1064 (HY000): errCode = 2, detailMessage = failed to execute delete. transaction id 7215554, timeout(ms) 160000, unfinished replicas: 4718319=7345841
```

We do not know the meaning of `4718319=7345841`.

Actually the former is `BackendId` and the latter is `TabletId`.

I'll add an instruction here to help locate the problem quickly. The error message will be 

```sql
ERROR 1064 (HY000): errCode = 2, detailMessage = failed to execute delete. transaction id 7215554, timeout(ms) 160000, unfinished replicas [BackendId=TabletId]: 4718319=7345841
```

fe/fe-core/src/main/java/org/apache/doris/load/DeleteHandler.java

@@ -284,7 +284,7 @@ public class DeleteHandler implements Writable {
                 // only show at most 5 results
                 List<Entry<Long, Long>> subList = unfinishedMarks.subList(0, Math.min(unfinishedMarks.size(), 5));
                 if (!subList.isEmpty()) {
-                    errMsg = "unfinished replicas: " + Joiner.on(", ").join(subList);
+                    errMsg = "unfinished replicas [BackendId=TabletId]: " + Joiner.on(", ").join(subList);
                 }
                 LOG.warn(errMsg);