Added --fail-on stopped-replication param to table checksum

I'm running pt-table-checksum using dsn table for discovering slaves but is not reporting a failing slave, either if not replicating (i.e. io/sql thread stopped) or if instance is down. 

This is the command I'm running: 
pt-table-checksum --replicate=percona.checksum --retries=3 --create-replicate-table --no-check-replication-filters --no-check-binlog-format --recursion-method dsn=h=127.0.0.1,D=percona,t=dsns --ignore-databases=mysql,percona --progress=time,10 --max-lag=600 h=127.0.0.1,P=3306,u=root,p=xxxxxxx