Error handling for gallery images

Invalid asset_ids results in error 500, should be 404

NoMethodError (undefined method `asset' for nil:NilClass):
  extensions/pages/gallery_page/app/controllers/gallery_image_controller.rb:10:in `show'

Edited Oct 16, 2018 by dgt

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information