Fix decoding of errors in SubDocGetRequest

After a top-level INVALID_REQUEST error, it was trying to decoding the json error message into a status, valueLength and value for each command. The status is invalid, and the valueLength is too long, resulting in an exception being thrown from body.readBytes(value, 0, valueLength), resulting in a DecodingFailureException. As I don't know the exact set of top-level rawStatus = status(body) that do not have the list of values returned, instead I check that the status for the 'value' element is 7b 22 "{ (start of a json error message).