I was able to successfully upgrade to 2.15, but after running overnight MySQL quit running and will not restart. I am getting the following error when checking the status of MySQL in Linux. 'Can’t connect to local MySQL server through socket ‘/tmp/mysql.sock’ Check that mysqld is running and that the socket: ‘/tmp/mysql.sock’ exists!. I also see the following error in the browser SQLSTATE[HY000]  Connection refused.
This is my second attempt to upgrade to this version with the same results.
Note: I am doing an incremental upgrade from version 1.12.