CurlEasyHandle::mErrorBuffer (CURLOPT_ERRORBUFFER) can NOT be
used to retrieve information about an error returned by
curl_multi_info_read in CURLMsg::data::result. This buffer is
only initialized when a curl_easy_* call returns an error,
and those errors are already printed automagically.
Initialize the buffer with an empty string upon invokation
of an curl_easy_* call, so we are sure the error belongs to
the last call.
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