Prevent global 404 on /attract when an activity can't be linked.

This happens when an activity is on a deleted item. This change implies
that "someone deleted X" activities aren't going to be shown.

attract/routes.py

@@ -3,6 +3,7 @@ import logging
 
 from flask import Blueprint, render_template, redirect, url_for
 import flask_login
+import werkzeug.exceptions as wz_exceptions
 
 from pillar.web.utils import attach_project_pictures
 import pillar.web.subquery
@@ -55,7 +56,7 @@ def index():
         act.project = id_to_proj[act.project]
         try:
             act.link = current_attract.link_for_activity(act)
-        except ValueError:
+        except (ValueError, wz_exceptions.NotFound):
             act.link = None
 
     return render_template('attract/index.html',