Using singular subject in exception message

The keys in the translation files are still the new 2.0 style (plural) but the subject being returned by the exception message shouldn't be plural. This makes this possible:

```ruby
en:
  unauthorized:
    access:
      all: 'You do not have permission to %{action} this %{subject}.'
```

Maybe your intent was to use plural subjects here (e.g. ... have permission to edit comments) but in general you're only asking if the user can edit a singular object, not if they can edit comments in general (instance vs class object).  If this is your intent I purpose a subject_plural key or similar as there is no way to run code on translation variables.