Developer UX: better error for misspelled/missing attribute

Created by: monkbroc

If a field in COLLECTION_ATTRIBUTES has no key in ATTRIBUTE_TYPES either because I misspelled it or I forget to add it to ATTRIBUTE_TYPES I will get a undefined method new for nil:NilClass in Page::Base

It would be nicer to see a custom exception Attribute xyz not found in dashboard AbcDashboard

This has already happened multiple times on my first day of using Administrate so I think this is worthwhile to fix.