Hi !
I had the same issue but I found a solution !
You need another computer under linux and put your pi sdcard in it.
If your RPI sdcard is /dev/sdb do :
sudo umount /dev/sdb1
sudo umount /dev/sdb2
sudo fsck /dev/sdb1
sudo fsck /dev/sdb2
After put the sdcard in the raspberry pi and enjoy 
But I think Ubuntu mate mainteners have to do something for that !