on-failure hook not triggering

I was doing a long-ish migration on a disk constrained server that had a limit on the number of binlog files available.

After some amount of lag the required binlog file was deleted and `binlogstreamer.go` failed because it couldn't find it.

The problem is that I have a on-failure hook configured that should have been triggered and it didn't, so the migration was interrupted and I did not get the notification to fix the problem and start over.

```
Copy: 31257000/336329667 9.3%; Applied: 40758; Backlog: 0/1000; Time: 1h52m15s(total), 1h52m5s(copy); streamer: mysql-bin.011888:4; State: throttled, lag=247.464956s; ETA: 18h13m56s
2018/11/29 11:56:17 binlogsyncer.go:79: [info] create BinlogSyncer with config {99999 mysql 10.0.200.156 3306 ghost false false }
2018/11/29 11:56:17 binlogsyncer.go:246: [info] begin to sync binlog from position (mysql-bin.011888, 4)
2018/11/29 11:56:17 binlogsyncer.go:139: [info] register slave for master server 10.0.200.156:3306
2018/11/29 11:56:17 binlogstreamer.go:47: [error] close sync with err: ERROR 1236 (HY000): Could not find first log file name in binary log index file
Copy: 31257000/336329667 9.3%; Applied: 40758; Backlog: 0/1000; Time: 1h52m20s(total), 1h52m10s(copy); streamer: mysql-bin.011888:4; State: throttled, lag=251.564998s; ETA: 18h14m45s
2018-11-29 11:56:22 FATAL 61 successive failures in streamer reconnect at coordinates mysql-bin.011888:4
```