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.go

@@ -1,5 +1,7 @@
 package errorsx
 
+import "encoding/json"
+
 // ErrWrapper is our error wrapper for Go errors. The key objective of
 // this structure is to properly set Failure, which is also returned by
 // the Error() method, to be one of the OONI failure strings.
@@ -60,3 +62,8 @@ func (e *ErrWrapper) Error() string {
 func (e *ErrWrapper) Unwrap() error {
 	return e.WrappedErr
 }
+
+// MarshalJSON converts an ErrWrapper to a JSON value.
+func (e *ErrWrapper) MarshalJSON() ([]byte, error) {
+	return json.Marshal(e.Failure)
+}

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)
+		}
+	})
 }