```ruby
if ::Rails::VERSION::MAJOR == 3 ||
    (::Rails::VERSION::MAJOR == 4 && ::Rails::VERSION::MINOR == 0)
  require 'i18n'

  # Override exception handler to more carefully html-escape missing-key results.
  class HtmlSafeI18nExceptionHandler
    def call(exception, locale, key, options)
      keys = exception.keys.map { |k| Rack::Utils.escape_html k }
      key = keys.last.to_s.gsub('_', ' ').gsub(/\b('?[a-z])/) { $1.capitalize }
      %(<span class="translation_missing" title="translation missing: #{keys.join('.')}">#{key}</span>)
    end
  end

  I18n.exception_handler = HtmlSafeI18nExceptionHandler.new
end
```

Update exception handler for dummy app
user
```ruby
if ::Rails::VERSION::MAJOR == 3 ||
    (::Rails::VERSION::MAJOR == 4 && ::Rails::VERSION::MINOR == 0)
  require 'i18n'

  # Override exception handler to more carefully html-escape missing-key results.
  class HtmlSafeI18nExceptionHandler
    Missing = I18n.const_defined?(:MissingTranslation) ? I18n::MissingTranslation : I18n::MissingTranslationData

    def initialize(original_exception_handler)
      @original_exception_handler = original_exception_handler
    end

    def call(exception, locale, key, options)
      if exception.is_a?(Missing) && options[:rescue_format] == :html
        keys = exception.keys.map { |k| Rack::Utils.escape_html k }
        key = keys.last.to_s.gsub('_', ' ').gsub(/\b('?[a-z])/) { $1.capitalize }
        %(<span class="translation_missing" title="translation missing: #{keys.join('.')}">#{key}</span>)
      else
        @original_exception_handler.call(exception, locale, key, options)
      end
    end
  end

  I18n.exception_handler = HtmlSafeI18nExceptionHandler.new(I18n.exception_handler)
end
```
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