Disk Partition failed -- invalid volume free block count
Have you tried running it a second time? Often with "File system check exit code is 8", others have had success repairing a second or third time. 

You may also try booting from the recovery partition and running doing the repair. You can boot from the recovery partition by restarting and holding down the Option key, then select the Recovery partition.

If you don't have a recovery partition, you can do an Iternet Recovery by hold down the Command Option R keys. 

Here is discussion on Apple's Communities forum.