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/dialer.go

@@ -0,0 +1,79 @@
+package errorsx
+
+import (
+	"context"
+	"net"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+)
+
+// Dialer establishes network connections.
+type Dialer interface {
+	// DialContext behaves like net.Dialer.DialContext.
+	DialContext(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// ErrorWrapperDialer is a dialer that performs error wrapping. The connection
+// returned by the DialContext function will also perform error wrapping.
+type ErrorWrapperDialer struct {
+	// Dialer is the underlying dialer.
+	Dialer
+}
+
+// DialContext implements Dialer.DialContext.
+func (d *ErrorWrapperDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	conn, err := d.Dialer.DialContext(ctx, network, address)
+	if err != nil {
+		return nil, SafeErrWrapperBuilder{
+			Classifier: errorsx.ClassifyGenericError,
+			Operation:  errorsx.ConnectOperation,
+			Error:      err,
+		}.MaybeBuild()
+	}
+	return &errorWrapperConn{Conn: conn}, nil
+}
+
+// errorWrapperConn is a net.Conn that performs error wrapping.
+type errorWrapperConn struct {
+	// Conn is the underlying connection.
+	net.Conn
+}
+
+// Read implements net.Conn.Read.
+func (c *errorWrapperConn) Read(b []byte) (int, error) {
+	count, err := c.Conn.Read(b)
+	if err != nil {
+		return 0, SafeErrWrapperBuilder{
+			Classifier: errorsx.ClassifyGenericError,
+			Operation:  errorsx.ReadOperation,
+			Error:      err,
+		}.MaybeBuild()
+	}
+	return count, nil
+}
+
+// Write implements net.Conn.Write.
+func (c *errorWrapperConn) Write(b []byte) (int, error) {
+	count, err := c.Conn.Write(b)
+	if err != nil {
+		return 0, SafeErrWrapperBuilder{
+			Classifier: errorsx.ClassifyGenericError,
+			Operation:  errorsx.WriteOperation,
+			Error:      err,
+		}.MaybeBuild()
+	}
+	return count, nil
+}
+
+// Close implements net.Conn.Close.
+func (c *errorWrapperConn) Close() error {
+	err := c.Conn.Close()
+	if err != nil {
+		return SafeErrWrapperBuilder{
+			Classifier: errorsx.ClassifyGenericError,
+			Operation:  errorsx.CloseOperation,
+			Error:      err,
+		}.MaybeBuild()
+	}
+	return nil
+}