Follow-up to #46605.

For a scenario like:

  ```ruby
  class Cat < ActiveRecord::Base
    has_many :lives
  end

  class Life
  end
  ```

this commit changes the error from:

> Rails couldn't find a valid model for the lives association. Use the
> :class_name option on the association declaration to tell Rails which
> model to use.

to:

> The Life model class for the Cat#lives association is not an
> ActiveRecord::Base subclass.

Additionally, for a scenario like:

  ```ruby
  class Cat < ActiveRecord::Base
    has_many :lives
  end

  class Live < ActiveRecord::Base
  end
  ```

this commit changes the error from:

> Rails couldn't find a valid model for the lives association. Use the
> :class_name option on the association declaration to tell Rails which
> model to use.

to:

> Missing model class Life for the Cat#lives association. You can
> specify a different model class with the :class_name option.