provide Response Code and Message if Server Error on Write

Current Behavior:
When performing a `write` operation in which an error of `>= 400` raises the ambiguous error `'Encountered server error'`.

Desired Behavior:
The desired behavior would be to provide the actual `response.code` and, if available, `response.message` or similar.

Line/Modification Requested:
Currently in master, in file fluent-plugin-logdna/lib/fluent/plugin/out_logdna.rb, line 44 reads `raise 'Encountered server error' if response.code >= 400`. Link: https://github.com/logdna/fluent-plugin-logdna/blob/6f28630169ee71380be28a75d6e245a4def3156f/lib/fluent/plugin/out_logdna.rb#L44