Fixed dumping json - added copying string workaround.

Updated json stat docs. 
I've already described the purpose. Yes it doesn't show free space error, network error and so on. But it shows the error while writing periodic statistics and it will do it even if no other operations are performed on disk.
You can't combine all possible errors in one place, in one value with saving its usefulness. This value covers certain part of possible errors and I believe that we can add another ones which will cover remaining errors.