fix(netxlite/errorsx): serialize directly to JSON (#491)

This simplifies serializing errors to `*string`. It did not
occur to me before. It seems quite a nice improvement.

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

internal/netxlite/errorsx/errwrapper_test.go

@@ -1,6 +1,7 @@
 package errorsx
 
 import (
+	"encoding/json"
 	"errors"
 	"io"
 	"testing"
@@ -23,4 +24,19 @@ func TestErrWrapper(t *testing.T) {
 			t.Fatal("cannot unwrap error")
 		}
 	})
+
+	t.Run("MarshalJSON", func(t *testing.T) {
+		wrappedErr := &ErrWrapper{
+			Failure:    FailureEOFError,
+			WrappedErr: io.EOF,
+		}
+		data, err := json.Marshal(wrappedErr)
+		if err != nil {
+			t.Fatal(err)
+		}
+		s := string(data)
+		if s != "\""+FailureEOFError+"\"" {
+			t.Fatal("invalid serialization", s)
+		}
+	})
 }