the laws of physics are reconsidered constantly anyway, this will be no inconvenience, it's what physicists do. However it's a bit of a leap to say if this fails then the theory is wrong. GWs are very difficult to detect, especially with the tiny amount of mass we have to work with. It may need super massive/sensitive space based equipment to detect the waves.
I suppose the point is that this time the experiment is expected to be sensitive enough at last that the possibility of detecting gravitational waves is greater than ever before, and the experimental errors small enough that it would be definitive one way or the other. Not checked the details -- I don't tend to care much about gravity in my work -- but if that is the case then a non-discovery would be a seriously big deal. Gravitational waves are a necessary consequence of GR, though, so it's pretty much a given that they will be detected at some point.