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)
+}