3.0 move exception

This is the first step towards fixing the use of exceptions in cake. Refs #4416

Next step would be to use more relevant exceptions instead of just throwing the generic cake one. When not using the extra features of this exception type, there is no point in using it.
@markstory No, I think it is a good idea, it is just that it took me a long time to rename the exception namespace and I had to do it again. It also mean that in other namespaces we will need to rename `Error` to `Exception`