Replace assertions in slice method with alert func

[NickEntin]

There are some valid use cases where you want to be able to slice more than the available width/height, such as when your content has a minimum size and your sizing rules allow for content to extend beyond the bounds if you view is sized beneath its ideal size. This replaces the assertions in the slice(...) method with alert functions consumers can place breakpoints for, following the pattern we use for view alignment issues.

Resolves #128