It seems like that it's trying to create an index but there is already one there, so it doesn't create the one it expects. And in the later step when its looking for the index, it doesn't find it and bombs.
Drop the key on the migrations_lock table and restart it.
It'll do the rest of the update and work again.
That's what I did with my test sites which doesn't have a lot of content and it seems to work.
But I *would* recommend a backup of what you have first though, include the db, just in case it updates something you don't want.
GL!
P.S. Ghost 4.0 is very different... You might want to check that out before you fully upgrade.
