netxlite: improve docs, tests, and code quality (#493)

* netxlite: improve docs, tests, and code quality

* better documentation

* more strict testing of dialer (especially make sure we
document the quirk in https://github.com/ooni/probe/issues/1779
and we have tests to guarantee we don't screw up here)

* introduce NewErrWrapper factory for creating errors so we
have confidence we are creating them correctly

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

internal/netxlite/errorsx/errwrapper_test.go

@@ -5,6 +5,8 @@ import (
 	"errors"
 	"io"
 	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/atomicx"
 )
 
 func TestErrWrapper(t *testing.T) {
@@ -40,3 +42,63 @@ func TestErrWrapper(t *testing.T) {
 		}
 	})
 }
+
+func TestNewErrWrapper(t *testing.T) {
+	t.Run("panics if the classifier is nil", func(t *testing.T) {
+		recovered := &atomicx.Int64{}
+		func() {
+			defer func() {
+				if recover() != nil {
+					recovered.Add(1)
+				}
+			}()
+			NewErrWrapper(nil, CloseOperation, io.EOF)
+		}()
+		if recovered.Load() != 1 {
+			t.Fatal("did not panic")
+		}
+	})
+
+	t.Run("panics if the operation is empty", func(t *testing.T) {
+		recovered := &atomicx.Int64{}
+		func() {
+			defer func() {
+				if recover() != nil {
+					recovered.Add(1)
+				}
+			}()
+			NewErrWrapper(ClassifyGenericError, "", io.EOF)
+		}()
+		if recovered.Load() != 1 {
+			t.Fatal("did not panic")
+		}
+	})
+
+	t.Run("panics if the error is nil", func(t *testing.T) {
+		recovered := &atomicx.Int64{}
+		func() {
+			defer func() {
+				if recover() != nil {
+					recovered.Add(1)
+				}
+			}()
+			NewErrWrapper(ClassifyGenericError, CloseOperation, nil)
+		}()
+		if recovered.Load() != 1 {
+			t.Fatal("did not panic")
+		}
+	})
+
+	t.Run("otherwise, works as intended", func(t *testing.T) {
+		ew := NewErrWrapper(ClassifyGenericError, CloseOperation, io.EOF)
+		if ew.Failure != FailureEOFError {
+			t.Fatal("unexpected failure")
+		}
+		if ew.Operation != CloseOperation {
+			t.Fatal("unexpected operation")
+		}
+		if ew.WrappedErr != io.EOF {
+			t.Fatal("unexpected WrappedErr")
+		}
+	})
+}