curl_easy_strerror(3) on macOS

The text 7fb13fc016f0, as Mac OS X 10.6.8, Mac OS X 10.6.7, Mac OS X 10.6.6, Mac OS X 10.6.5, Mac OS X 10.6.4, Mac OS X 10.6.3 shipped it.

return string describing error code

curl_easy_strerror(3) on macOS now