Instead of treating it as an anonymous block, execute the
`ActionView::Base.field_error_proc` within the context of the
`ActionView::Base` instance.

This enables consumer applications to continue to override the proc as
they see fit, but frees them from declaring templating logic within a
`config/initializers/*.rb`, `config/environments/*.rb` or
`config/application.rb` file.

This makes it possible to replace something like:

```ruby
config.action_view.field_error_proc = proc do |html_tag, instance|
  <<~HTML.html_safe
    #{html_tag}
    <span class="errors">#{instance.error_message.to_sentence}</span>
  HTML
end
```

With inline calls to Action View helpers like:

```ruby
config.action_view.field_error_proc = proc do |html_tag, instance|
  safe_join [ html_tag, tag.span(instance.error_message.to_sentence, class: "errors") ]
end
```

Or with a view partial rendering, like:

```ruby
config.action_view.field_error_proc = proc do |html_tag, instance|
  render partial: "application/field_with_errors", locals: { html_tag: html_tag, instance: instance }
end
```

Then, elsewhere in `app/views/application/field_with_errors.html.erb`:

```erb
<%= html_tag %>
<span class="errors"><%= instance.error_message.to_sentence %></span>
```