Comment 13 for bug 1574168

Repost from duplicate bug thread: Also receiving this error. A ton of errors really. It also failed to update several MySQL tables in information_schema and performance_schema and many other things. Over a series of hours I got MySQL to work by running mysql_upgrade and doing several other things, some of which are linked to below, sorry I did not create exact steps:

http://askubuntu.com/questions/705458/ubuntu-15-10-mysql-error-1524-unix-socket
http://stackoverflow.com/questions/31967527/table-performance-schema-session-variables-doesnt-exist

Finally I noticed this:
Setting up mysql-server-5.7 (5.7.12-0ubuntu1.1) ...
mysql_upgrade: Got error: 1045: Access denied for user 'debian-sys-maint'@'localhost' (using password: YES) while connecting to the MySQL server

SOLUTION:
CREATE USER 'debian-sys-maint'@'localhost' IDENTIFIED BY '***';GRANT ALL PRIVILEGES ON *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY '***' REQUIRE NONE WITH GRANT OPTION MAX_QUERIES_PER_HOUR 0 MAX_CONNECTIONS_PER_HOUR 0 MAX_UPDATES_PER_HOUR 0 MAX_USER_CONNECTIONS 0;

Note: You can get the debian-sys-maint password from /etc/mysql/debian.cnf

Then: sudo apt-get upgrade

Anyways good luck folks, this took me several hours to move through this hell. I believe something messed up with the install, the debian-sys-maint user was not created, so system could not update MySQL schema which probably lead to a whole host of other errors. Be sure to check your logs so you can resolve this quicker than it took me.