fix(netxlite/errorsx): map "use of closed network connection" (#510)

Part of https://github.com/ooni/probe/issues/1733 and diff has been
extracted from https://github.com/ooni/probe-cli/pull/506.

internal/netxlite/errorsx/classify.go

@@ -90,6 +90,9 @@ func classifyWithStringSuffix(err error) string {
 		// that we return here is significantly more specific.
 		return FailureDNSNXDOMAINError
 	}
+	if strings.HasSuffix(s, "use of closed network connection") {
+		return FailureConnectionAlreadyClosed
+	}
 	return "" // not found
 }