Rename error messages for restrict_dependent_destroy errors

Previously `has_one` and `has_many` associations were using the `one` and `many` messages respectively.

Both of these keys have special meaning in I18n (they are considered to be pluralizations) so by renaming them to `has_one` and `has_many` we make the messages more explicit and most importantly they don't clash with linguistical systems that need to validate translation keys (and their pluralizations).