refactor(i/errorsx): always use the same error reporting pattern (#478)

For consistency and also because the SafeErrorWrapperBuilder seems
to be the building pattern than the original code assumed.

New code should not use it, but I'd rather keep legacy code consistent
formally and with its own original assumptions.

In particular, it matters that SafeErrorWrapperBuilder assigns the
most relevant operation that failed. We were not doing that when we
were manually creating a new ErrWrapper.

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

internal/errorsx/dialer.go

@@ -24,11 +24,11 @@ type ErrorWrapperDialer struct {
 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, &errorsx.ErrWrapper{
-			Failure:    errorsx.ClassifyGenericError(err),
+		return nil, SafeErrWrapperBuilder{
+			Classifier: errorsx.ClassifyGenericError,
 			Operation:  errorsx.ConnectOperation,
-			WrappedErr: err,
-		}
+			Error:      err,
+		}.MaybeBuild()
 	}
 	return &errorWrapperConn{Conn: conn}, nil
 }
@@ -43,11 +43,11 @@ type errorWrapperConn struct {
 func (c *errorWrapperConn) Read(b []byte) (int, error) {
 	count, err := c.Conn.Read(b)
 	if err != nil {
-		return 0, &errorsx.ErrWrapper{
-			Failure:    errorsx.ClassifyGenericError(err),
+		return 0, SafeErrWrapperBuilder{
+			Classifier: errorsx.ClassifyGenericError,
 			Operation:  errorsx.ReadOperation,
-			WrappedErr: err,
-		}
+			Error:      err,
+		}.MaybeBuild()
 	}
 	return count, nil
 }
@@ -56,11 +56,11 @@ func (c *errorWrapperConn) Read(b []byte) (int, error) {
 func (c *errorWrapperConn) Write(b []byte) (int, error) {
 	count, err := c.Conn.Write(b)
 	if err != nil {
-		return 0, &errorsx.ErrWrapper{
-			Failure:    errorsx.ClassifyGenericError(err),
+		return 0, SafeErrWrapperBuilder{
+			Classifier: errorsx.ClassifyGenericError,
 			Operation:  errorsx.WriteOperation,
-			WrappedErr: err,
-		}
+			Error:      err,
+		}.MaybeBuild()
 	}
 	return count, nil
 }
@@ -69,11 +69,11 @@ func (c *errorWrapperConn) Write(b []byte) (int, error) {
 func (c *errorWrapperConn) Close() error {
 	err := c.Conn.Close()
 	if err != nil {
-		return &errorsx.ErrWrapper{
-			Failure:    errorsx.ClassifyGenericError(err),
+		return SafeErrWrapperBuilder{
+			Classifier: errorsx.ClassifyGenericError,
 			Operation:  errorsx.CloseOperation,
-			WrappedErr: err,
-		}
+			Error:      err,
+		}.MaybeBuild()
 	}
 	return nil
 }

internal/errorsx/quic.go

@@ -87,13 +87,12 @@ func (d *ErrorWrapperQUICDialer) DialContext(
 	ctx context.Context, network string, host string,
 	tlsCfg *tls.Config, cfg *quic.Config) (quic.EarlySession, error) {
 	sess, err := d.Dialer.DialContext(ctx, network, host, tlsCfg, cfg)
-	err = SafeErrWrapperBuilder{
-		Classifier: errorsx.ClassifyQUICHandshakeError,
-		Error:      err,
-		Operation:  errorsx.QUICHandshakeOperation,
-	}.MaybeBuild()
 	if err != nil {
-		return nil, err
+		return nil, SafeErrWrapperBuilder{
+			Classifier: errorsx.ClassifyQUICHandshakeError,
+			Error:      err,
+			Operation:  errorsx.QUICHandshakeOperation,
+		}.MaybeBuild()
 	}
 	return sess, nil
 }