refactor: i/errorsx is now i/legacy/errorsx (#479)

We need still to add similar wrappers to internal/netxlite but we
will adopt a saner approach to error wrapping this time.

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

internal/engine/legacy/errorsx/errorsx.go

@@ -0,0 +1,70 @@
+// Package errorsx contains error extensions.
+package errorsx
+
+import (
+	"errors"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+)
+
+// SafeErrWrapperBuilder contains a builder for ErrWrapper that
+// is safe, i.e., behaves correctly when the error is nil.
+type SafeErrWrapperBuilder struct {
+	// Error is the error, if any
+	Error error
+
+	// Classifier is the local error to string classifier. When there is no
+	// configured classifier we will use the generic classifier.
+	Classifier func(err error) string
+
+	// Operation is the operation that failed
+	Operation string
+}
+
+// MaybeBuild builds a new ErrWrapper, if b.Error is not nil, and returns
+// a nil error value, instead, if b.Error is nil.
+func (b SafeErrWrapperBuilder) MaybeBuild() (err error) {
+	if b.Error != nil {
+		classifier := b.Classifier
+		if classifier == nil {
+			classifier = errorsx.ClassifyGenericError
+		}
+		err = &errorsx.ErrWrapper{
+			Failure:    classifier(b.Error),
+			Operation:  toOperationString(b.Error, b.Operation),
+			WrappedErr: b.Error,
+		}
+	}
+	return
+}
+
+func toOperationString(err error, operation string) string {
+	var errwrapper *errorsx.ErrWrapper
+	if errors.As(err, &errwrapper) {
+		// Basically, as explained in ErrWrapper docs, let's
+		// keep the child major operation, if any.
+		if errwrapper.Operation == errorsx.ConnectOperation {
+			return errwrapper.Operation
+		}
+		if errwrapper.Operation == errorsx.HTTPRoundTripOperation {
+			return errwrapper.Operation
+		}
+		if errwrapper.Operation == errorsx.ResolveOperation {
+			return errwrapper.Operation
+		}
+		if errwrapper.Operation == errorsx.TLSHandshakeOperation {
+			return errwrapper.Operation
+		}
+		if errwrapper.Operation == errorsx.QUICHandshakeOperation {
+			return errwrapper.Operation
+		}
+		if errwrapper.Operation == "quic_handshake_start" {
+			return errorsx.QUICHandshakeOperation
+		}
+		if errwrapper.Operation == "quic_handshake_done" {
+			return errorsx.QUICHandshakeOperation
+		}
+		// FALLTHROUGH
+	}
+	return operation
+}