Map `CURLE_COULDNT_CONNECT` to `NSURLErrorCannotConnectToHost`.
When connecting to an IPv6 socket fails, curl easy will return `CURLE_COULDNT_CONNECT`. GNUstep currently maps `easyCode == CURLE_COULDNT_CONNECT && failureErrno == ETIMEDOUT` to `NSURLErrorTimedOut`, in all other scenarios `CURLE_COULDNT_CONNECT` is not handled.
This would cause the `NSURLSession/test01` to fail on (certain) systems with IPv6 enabled.
This PR maps all other values of `CURLE_COULDNT_CONNECT` to `NSURLErrorCannotConnectToHost`.
Additionally, it also stores the value of `easyCode` in the `NSUnderlyingErrorKey` to make troubleshooting (slightly) easier.