Comment 11 for bug 1567884

It's actually worse than that, because:
  (1) apt leaves the package dangling, not fully installed, requires a fix with "apt-get install -f" ... and
  (2) it leaves the databases and tables in the old pre-5.6 format because it didn't finish installation

So it's a bit of a mess. After manually fixing the my.cnf problem and doing a -f, it goes ahead and converts the databases and tables as well and finishes.

It's not Ubuntu's responsibility to fix the my.cnf problem, but maybe Ubuntu could allow some easy way to revert to 5.6 after a failed 5.7 upgrade, because right now it's a little scary for those who don't have much expertise to fix it themselves.