refactor(netxlite/errorsx): extract string-suffix classifier (#482)

This change makes the code more tidy and easier to read.

No functional change, though.

See https://github.com/ooni/probe/issues/1591.

internal/netxlite/errorsx/errno.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// Generated: 2021-09-07 20:26:06.502417 +0200 CEST m=+0.133209876
+// Generated: 2021-09-07 20:42:20.172357 +0200 CEST m=+0.135433792
 
 package errorsx
 
@@ -64,9 +64,6 @@ const (
 // proper OONI error. Returns the OONI error string
 // on success, an empty string otherwise.
 func classifySyscallError(err error) string {
-	// filter out system errors: necessary to detect all windows errors
-	// https://github.com/ooni/probe/issues/1526 describes the problem
-	// of mapping localized windows errors.
 	var errno syscall.Errno
 	if !errors.As(err, &errno) {
 		return ""