Proper error control

Hi @d11 
Using pull-request for easier coomunication when merging, in case I'm busy when you return. Please merge once you've seen it. I've just implemented the error control we discussed. Surprisingly simple! 

Check out the unit tests for error raising: the torch.Tester:assertError() method is very useful for that -- I added it in torch/torch7-distro#92 back in January, I'm glad to get to use it ;-)