br: the err message somtimes could be too large

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

restore a cluster with schema count large enough.
Trigger the err at (https://github.com/pingcap/tidb/blob/master/br/pkg/task/stream.go#L1864).
Because it will print all the schemas used here, the log would become too large to read.

*** Sometimes it would stuck the kubenates due to log length limit***
(ref:https://github.com/kubernetes/kubernetes/blob/db1990f48b92d603f469c1c89e2ad36da1b74846/test/integration/master/synthetic_master_test.go#L315)

### 2. What did you expect to see? (Required)

A smaller log.

### 3. What did you see instead (Required)

A too large log.

### 4. What is your TiDB version? (Required)

master