Add error stream API to EventRouter

With @ERROR, emitted events are routed to @ERROR label with warning message.
Without @ERROR, emitted events are dumpped in Fluentd's warning logs.
Use emit_error_event for each filter call
Add emit_error_event to RootAgentProxyWithoutErrorCollector
Merge pull request #492 from fluent/add-emit-error-event

Add error stream API to EventRouter