I find that the taps you find at most parts stores aren't very good. If you're just a little crooked or bind it up they'll snap like a twig. They do make an extractor to remove broken taps if isn't in too deep. Your not going to like this but the best thing to do is to remove the crank and take it to a machine shop that has an EDM machine and have them burn it out. Auto Auto machine shops don't have these machines. Your best bet would be to try a general purpose machine shop.
If you feeling really lucky and have some high quality small punches and chisels you can try to break the tap into pieces and dig them out of the hole. Try to spin the tap to the left with you chisel blows, sometimes you can get them to back out when you tap on them. Now your threads will really be screwed a Heli-coil and new bolt will be needed for sure.