curl_easy_strerror(3) on macOS

The text c00cc54bbc29, as Mac OS X 10.7.5, Mac OS X 10.7.4, Mac OS X 10.7.3, Mac OS X 10.7.2, Mac OS X 10.7.1, Mac OS X 10.7 shipped it.

return string describing error code

curl_easy_strerror(3) on macOS now