Dlc 17 - Listings UI [IN-PROGRESS]

@Saityi make a pull request next time and tag me. Don't be shy :)
rather than make this new file that is almost identical to _error_messages.html.erb, just further generalize _error_messages.html.erb to check @validated_entity for errors (instead of a particular kind of entity) and set it correctly in the handlers/actions that should trigger this page