Change errors and metadata accessor methods

This adds support for `store_errors` and `store_metadata`, based on [this comment thread](https://github.com/remiprev/her/commit/3c1391c3e4dacba4b24a121ed16767077fbef279#commitcomment-3038365) started by @pencil.

Basically, it allows us to do this:

```ruby
class User
  include Her::Model

  store_errors :errors # default is :response_errors
  store_metadata :my_data # default is :metadata
end
```

and override the `errors` method provided by `ActiveModel::Validations` and make it returns the value of `@response_errors`, which is the errors returned by the API.