When I use tar with the -v option I redirect the output to a log file, e.g.
The 2>&1 part redirects all errors to the log as well.
tar -cvpzf backup.tar.gz --exclude=/backup.tar.gz --exclude=/proc --exclude=/lost+found --exclude=/sys --exclude=/mnt --exclude=/media --exclude=/dev / > /var/log/mybackup.log 2>&1
The error 'tar: Exiting with failure status due to previous errors' is probably down to it not being able to archive a file or directory. In which case it should report it and then skip it, continuing on only to report that it had encountered an error at the end of the tar.
The log file should allow you to see what the error was on so that you can check yourself whether or not the error is serious or not.