Dropped events show up as errors on the reponse queue

[vreynolds]

Initially discovered in honeycombio/agentless-integrations-for-aws#96

When libhoney drops an event due to sampling, it puts an error on the response queue "event dropped due to sampling". This is confusing on the receiving end, because it's not really an error, nothing went wrong.

It seems this was implemented as an error, because of the limitation of the Response struct -- there's not really a better field to add the message that event was sampled out

• can we change the libhoney to communicate a sampled out event in a non-error way?
• alternatively, consumers like the agentless integration can be aware of this special case, and have special handling for sampling "errors"