Rats! But I think Paddy's solution of moving, even deleting the stuff you have will "convince" the App Store to let you try again. And I think you have determined that you weren't charged for that first attempt, correct? Since what you have is not workable, for whatever reason, I don't see much use in keeping it around.
I suppose the good news is that the installer is 'smart' enough to know the rest of the file is damaged. Better to know that now than have the machine locked upbecause of bad code. But it might help if the Store waited for conformation that a download was successful
before recording that it was complete. I'm not sure why anyone should be limited in downloading if they are verified as previous, paid up owners! Frustrating, in my opinion and not very 'customer' friendly, much less user friendly.
I hope it works correctly for you this time, please let us know how it goes.

I wasn't charged for that first attempt.
So I think I'll do a search for anything Lion and see what comes up, then trash the first "Max OS Install Lion.app", then go to the Apps Store and see if I can DL it again.
Thanks XABD.