Error notifications for admins at WSO

###### https://www.pivotaltracker.com/s/projects/982890/stories/65110172

- add gem `exception_notification` to handle notifications when errors ocurr in a Rails app.
- run `rails g exception_notification:install` to generate initializer.
- add code to `custom_errors.rb`
- add cucumber test to `customerrors.feature` 
- add rspec code to `custom_errors_spec.rb`
- refactoring `custom_errors_spec.rb`
- removed raise errors for deprecated numeric IDs.
- Cukes and Rspec are green