Standardize on CookbookNotFound errors

Fixes #1041
Standardize on DependencyNotFound
Move DuplicateDependency error text into errors.rb
Move InsufficientPrivledges error text into errors.rb
Move CommunitySiteError error text into errors.rb
Remove unused error classes
Standardize on what an error looks like
Fix failing cuke
Remove Berkshelf:: prefix on errors