[Reviewer: Andy] Take out fail on error flag

Can you review this change to no longer set the fail on error flag
Fixes #211 
This treats `3XX` responses as successes (unless libCurl is doing the redirect for us?) I think we could do with a utility function (similar to `curl_code_to_http_code`) that tells us if a request succeeded (taking the curl RC and the HTTP RC into consideration).  Thoughts?