I think you have the option to either proceed with the full amount or proceed with whatever is made. You have to make that choice at the start. Though that might be only Indiegogo?!
This campaign made (over?) 50% of the target figure. Would it not be possible to keep working on the game and keep promoting it any way possible and then re-run the campaign? Try to build on that 50%? There is certainly nothing preventing the re-running of the campaign on Indiegogo. Though Indiegogo only does a fraction of Kickstarters business. The 50% represents proof that there is interest in the project out there. That has to be worth something? No?
On a side note…..
There is also quite a bit of talk these days about crowd funding offering equity in projects. I know this is being looked into currently. They just need to find a way of managing the whole thing. A revenue split for funders would be very attractive indeed. If it ever comes to pass I think it’s a model that’ll suit indie games funding down to the ground.