added log debug message

revised and added log.debug message for setting log level debug and easy for users to see more detailed debug messages.
`#{!}` should be fixed to `#{$!}`, but now I think

```
rescue => e
  log.warn "out_mail: failed to send notice to #{@host}:#{@port}, subject: #{subject}, message: #{msg}, error_class: #{e.class}, error_message: #{e.message}, error_backtrace: #{e.backtrace.first}"
```

is better.