Fixed dumping json - added copying string workaround.

Updated json stat docs. 
If so, what is the purpose of saying that it failed to update data.stat file?

It is useless - if filesystem is in readonly mode, other operations will fail too - they will pop up RO error. But if there is no free space, this error will say nothing, but write operations will, and one has to have some mechanism to check those errors.

Do not add chunks which do not bring value. This error status and timestamp do not bring real value.