refactor(netxlite/errors): improve docs and format code (#481)

No real functional change. A few are needed and they will come
next. With this diff I just wanted to do cosmetic changes and
documentation changes, to ensure this package is okay.

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

internal/netxlite/errorsx/errwrapper_test.go

@@ -6,19 +6,21 @@ import (
 	"testing"
 )
 
-func TestErrWrapperError(t *testing.T) {
-	err := &ErrWrapper{Failure: FailureDNSNXDOMAINError}
-	if err.Error() != FailureDNSNXDOMAINError {
-		t.Fatal("invalid return value")
-	}
-}
+func TestErrWrapper(t *testing.T) {
+	t.Run("Error", func(t *testing.T) {
+		err := &ErrWrapper{Failure: FailureDNSNXDOMAINError}
+		if err.Error() != FailureDNSNXDOMAINError {
+			t.Fatal("invalid return value")
+		}
+	})
 
-func TestErrWrapperUnwrap(t *testing.T) {
-	err := &ErrWrapper{
-		Failure:    FailureEOFError,
-		WrappedErr: io.EOF,
-	}
-	if !errors.Is(err, io.EOF) {
-		t.Fatal("cannot unwrap error")
-	}
+	t.Run("Unwrap", func(t *testing.T) {
+		err := &ErrWrapper{
+			Failure:    FailureEOFError,
+			WrappedErr: io.EOF,
+		}
+		if !errors.Is(err, io.EOF) {
+			t.Fatal("cannot unwrap error")
+		}
+	})
 }