Add detailed error messages (ex: dimension mismatch)

[oir]

Should include which objects mismatch and their shapes.

I was thinking to leave this to Eigen itself since you get errors in mismatches but they are hard assertions, so this makes sense. However until you call forward you don't know the shapes of inputs, therefore you won't see the error at the line where you define the node. Is that okay?

Alternative is to leave this to Eigen, and switch eigen assertions to exceptions, which we already to for Cpu only builds. For GPUs however exceptions break nvcc (or at least, used to).