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/quirks_test.go

@@ -35,12 +35,16 @@ func TestQuirkReduceErrors(t *testing.T) {
 
 	t.Run("multiple errors with meaningful ones", func(t *testing.T) {
 		err1 := errors.New("mocked error #1")
-		err2 := &errorsx.ErrWrapper{
-			Failure: "unknown_failure: antani",
-		}
-		err3 := &errorsx.ErrWrapper{
-			Failure: errorsx.FailureConnectionRefused,
-		}
+		err2 := errorsx.NewErrWrapper(
+			errorsx.ClassifyGenericError,
+			errorsx.CloseOperation,
+			errors.New("antani"),
+		)
+		err3 := errorsx.NewErrWrapper(
+			errorsx.ClassifyGenericError,
+			errorsx.CloseOperation,
+			errorsx.ECONNREFUSED,
+		)
 		err4 := errors.New("mocked error #3")
 		result := quirkReduceErrors([]error{err1, err2, err3, err4})
 		if result.Error() != errorsx.FailureConnectionRefused {