Fixed dumping json - added copying string workaround.

Updated json stat docs. 
What is the real problem you are trying to solve?

The. Real. Problem.

Did you ever encounter a situation, where data.stat update error was ever needed? What was it?
Where and why do you need to read this error? How will you process it?

Our json stats is several megabytes large, it is supposed to be lightweight to read once per several second.
I do not accept patches which add more stuff like that just because 'it could be a good idea to have this error here'.
No, that is not how things have to be done. You have a real problem and solution for that - that's the way to update this codebase.