cbbackup - better handling of invalid metadata .json files

If a full-mode backup is interrupted, while writing .json files to disk, the .json files may not be well-formed. If cbbackup is run after that, with accu mode, it will try to read those files and fail. A Python stack trace is generated and the process hangs until you press Control-C. An improvement would be to handle the case of malformed .json files and to exit with an error message instead of hanging.