Lock screen authorisation error and flickering during release upgrade won't let me continue release upgrade from 15.04 to 16.x

[Nathan]

https://bugs.launchpad.net/ubuntu/+source/gdm/+bug/1414707

http://askubuntu.com/questions/762566/unable-to-login-to-gnome-session-during-release-upgrade

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.14
ProcVersionSignature: Ubuntu 4.2.0-35.40-generic 4.2.8-ckt5
Uname: Linux 4.2.0-35-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CrashDB: ubuntu
Date: Thu May 5 15:18:42 2016
InstallationDate: Installed on 2016-05-02 (2 days ago)
InstallationMedia: Ubuntu-GNOME 15.10 "Wily Werewolf" - Release amd64 (20151021)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_AU:en
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_AU.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
Symptom: release-upgrade
UpgradeStatus: Upgraded to xenial on 2016-05-05 (0 days ago)