curl_share_strerror(3) on macOS

The text 503edae395f6, 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_share_strerror(3) on macOS now