Make pt-online-schema-change exit with different codes depending on the status

https://www.percona.com/doc/percona-toolkit/LATEST/pt-online-schema-change.html

Exit codes for pt-online-schema-change are not documented. Generally at least a notice that non-zero exit codes indicating an error. The best option is an ability to distinguish between:

• incorrect invocation
• mysql connection errors
• network interrupts
• replication lag/stop