Change debug log level to warn in in_syslog

Pattern mismatch log should be warn for early detection.
We can disable this message by `log_level error` in production.

https://groups.google.com/d/msg/fluentd/ZRreaaRFR2U/ineHN8FOGI0J
`emit` catches `Engine.emit` error so this rescue catches another exception.
So we should dump the message as `error` level.