Improve backup operation progress tracking

The current "fetch-cmd-result" command only returns a human-readable string, lacking details. This is an issue with backup commands as these can take a long time without the user knowing what is going on. Add another command, "fetch-cmd-status" which returns a json array with timestamped status update elements. These are more akin to log entries than a simple status message. Such entries can be printed in the GUI when running a backup operation.

Also, improve Mariabackup error message handling so that multiline error messages are returned as a json string array instead of encoded into a single json string.