Improve error handling for unknown sprite icons

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Summary

See this thread for context.

Currently, in the icons_helper.rb we will throw an error on non-production environments if an icon is not found.

  def known_sprites
    return if Rails.env.production?

    @known_sprites ||= Gitlab::Json.parse(File.read(Rails.root.join('node_modules/@gitlab/svgs/dist/icons.json')))['icons']
  end

  def known_file_icon_sprites
    return if Rails.env.production?

    @known_file_icon_sprites ||= Gitlab::Json.parse(File.read(Rails.root.join('node_modules/@gitlab/svgs/dist/file_icons/file_icons.json')))['icons']
  end

Ideally, we should avoid the production check and see if there's a more graceful way to fail regardless of the environment.