Non-deprecated replacement w/ Cookbook#deprecate

If a deprecated cookbook is attempted to be set as the replacement w/ the
Cookbook#deprecate method, do not allow it.
Display errors when Cookbook deprecation fails.

This renders the errors on @cookbook within the CookbooksController#deprecate
action. It also adds new specs to test this, as well as adjusting the existing
specs to be more within the scope of controller tests.