You don't seem to have any of the recommended meta-packages installed:
2015-03-07 18:11:28,359 DEBUG none of the '['ubuntu-desktop', 'kubuntu-desktop', 'xubuntu-desktop', 'ubuntustudio-desktop', 'mythbuntu-desktop', 'kubuntu-netbook', 'lubuntu-desktop', 'ubuntu-gnome-desktop']' meta-pkgs installed 2015-03-07 18:11:28,359 DEBUG guessing 'ubuntu-gnome-desktop' as missing meta-pkg
If you install the ubuntu-gnome-desktop meta package on your system I suspect the release upgrade will succeed.
You don't seem to have any of the recommended meta-packages installed:
2015-03-07 18:11:28,359 DEBUG none of the '['ubuntu-desktop', 'kubuntu-desktop', 'xubuntu-desktop', 'ubuntustudio- desktop' , 'mythbuntu- desktop' , 'kubuntu-netbook', 'lubuntu-desktop', 'ubuntu- gnome-desktop' ]' meta-pkgs installed gnome-desktop' as missing meta-pkg
2015-03-07 18:11:28,359 DEBUG guessing 'ubuntu-
If you install the ubuntu- gnome-desktop meta package on your system I suspect the release upgrade will succeed.