refactor(errorsx): prepare for splitting the package (#476)

We will move the sane part of this package to i/netxlite/errorsx
and we will move the rest to i/e/legacy/errorsx.

What is the sane part? The sane part is error classifiers plus
the definition of ErrWrapper. The rest, including the rules
on how to decide whether an operation is major, are tricky and
we should consider them legacy and replace them with rules
that are more easy to understand and reason on.

Part of https://github.com/ooni/probe/issues/1591

internal/errorsx/tls.go

@@ -25,15 +25,16 @@ func (h *ErrorWrapperTLSHandshaker) Handshake(
 ) (net.Conn, tls.ConnectionState, error) {
 	tlsconn, state, err := h.TLSHandshaker.Handshake(ctx, conn, config)
 	err = SafeErrWrapperBuilder{
-		Classifier: classifyTLSFailure,
+		Classifier: ClassifyTLSHandshakeError,
 		Error:      err,
 		Operation:  TLSHandshakeOperation,
 	}.MaybeBuild()
 	return tlsconn, state, err
 }
 
-// classifyTLSFailure is a classifier to translate TLS errors to OONI error strings.
-func classifyTLSFailure(err error) string {
+// ClassifyTLSHandshakeError maps an error occurred during the TLS
+// handshake to an OONI failure string.
+func ClassifyTLSHandshakeError(err error) string {
 	var x509HostnameError x509.HostnameError
 	if errors.As(err, &x509HostnameError) {
 		// Test case: https://wrong.host.badssl.com/
@@ -50,5 +51,5 @@ func classifyTLSFailure(err error) string {
 		// Test case: https://expired.badssl.com/
 		return FailureSSLInvalidCertificate
 	}
-	return toFailureString(err)
+	return ClassifyGenericError(err)
 }