Ubuntu upgrade failed

login as: root
<email address hidden>'s password:
Welcome to Ubuntu 14.04.4 LTS (GNU/Linux 3.12.27-xenU x86_64)

 * Documentation: https://help.ubuntu.com/
New release '16.04.1 LTS' available.
Run 'do-release-upgrade' to upgrade to it.

*** /dev/xvda2 should be checked for errors ***

Last login: Sat Jul 30 11:54:01 2016 from 104-59-238-130.lightspeed.rlghnc.sbcgl obal.net
root@www5b:~# do-release-upgrade
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [198 B]
Get:2 Upgrade tool [1265 kB]
Fetched 1265 kB in 0s (0 B/s)
authenticate 'xenial.tar.gz' against 'xenial.tar.gz.gpg'
extracting 'xenial.tar.gz'

updates of cacerts keystore disabled.
done.
Processing triggers for libgdk-pixbuf2.0-0:amd64 (2.32.2-1ubuntu1) ...
Errors were encountered while processing:
 mysql-server-5.7
 mysql-server
Error in function:

A fatal error occurred

Please report this as a bug and include the files
/var/log/dist-upgrade/main.log and /var/log/dist-upgrade/apt.log in
your report. The upgrade has aborted.
Your original sources.list was saved in
/etc/apt/sources.list.distUpgrade.

SystemError: E:Sub-process /usr/bin/dpkg returned an error code (1)

Could not install the upgrades

The upgrade has aborted. Your system could be in an unusable state. A
recovery will run now (dpkg --configure -a).

Please report this bug in a browser at
http://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+filebug
and attach the files in /var/log/dist-upgrade/ to the bug report.
installArchives() failed

Setting up mysql-server-5.7 (5.7.13-0ubuntu0.16.04.2) ...
Renaming removed key_buffer and myisam-recover options (if present)
ERROR: Unable to start MySQL server:
2016-07-30T12:32:44.042843Z 0 [ERROR] unknown variable 'character_set_client=utf8'
2016-07-30T12:32:44.047735Z 0 [ERROR] Aborting
Please take a look at https://wiki.debian.org/Teams/MySQL/FAQ for tips on fixing common upgrade issues.
Once the problem is resolved, run apt-get --fix-broken install to retry.
dpkg: error processing package mysql-server-5.7 (--configure):
 subprocess installed post-installation script returned error exit status 1
dpkg: dependency problems prevent configuration of mysql-server:
 mysql-server depends on mysql-server-5.7; however:
  Package mysql-server-5.7 is not configured yet.

dpkg: error processing package mysql-server (--configure):
 dependency problems - leaving unconfigured
Errors were encountered while processing:
 mysql-server-5.7
 mysql-server

Upgrade complete

The upgrade has completed but there were errors during the upgrade
process.

To continue please press [ENTER]