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
Use quotes instead of weird backtacks
Merge pull request #1051 from berkshelf/sethvargo/better_errors

Improve errors