Can't open the mysql.plugin table error when installing MySQL on Mac OS Sierra
In my case mysql failed after the update because the user running mysqld (The user mysql) being unable to open the mysql data directory with write permission. The files were instead owned by myself. I fixed this with:

sudo chown -R mysql /usr/local/mysql/*


The orignal cause (for me) was not the Sierra upgrade itelf, but the overall process of updating applications in response. Specifically other instructions online for updating PHP with brew after the Sierra update had included a chown:

sudo chown -R $(whoami):admin /usr/local # DO NOT DO THIS


This had unfortunately altered the file permissions within /usr/local/mysql within /usr/local, causing the error.