Display helpful message if attribute_name missing

Created by: 5minpause

Problem

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

Solution

Don't access hash values directly but use fetch. If the attribute is not found the block raises an error with a helpful error message.

Closes #246 (closed)