return 404 where it makes sense

app/routes/topics.py

@@ -1,5 +1,6 @@
 from flask import (
-    Blueprint, render_template, request, redirect, url_for, flash, session
+    Blueprint, render_template, request, redirect, url_for, flash, session,
+    abort,
     )
 from .users import login_required, mod_only, get_active_user, is_logged_in
 from ..models import Users, Topics, Threads, Subscriptions
@@ -50,7 +51,8 @@ def topic(slug):
         "slug": slug
     })
     if not target_topic:
-        return redirect(url_for('.all_topics'))
+        abort(404)
+        return
 
     threads_count = Threads.count({
         "topic_id": target_topic.id
@@ -88,7 +90,8 @@ def topic(slug):
 def edit(slug):
     topic = Topics.find({"slug": slug})
     if not topic:
-        return redirect(url_for('.all_topics'))
+        abort(404)
+        return
     return render_template("topics/edit.html", topic=topic)
 
 
@@ -98,7 +101,8 @@ def edit(slug):
 def edit_post(slug):
     topic = Topics.find({"slug": slug})
     if not topic:
-        return redirect(url_for('.all_topics'))
+        abort(404)
+        return
 
     topic.update({
         "name": request.form.get('name', default = topic.name).strip(),
@@ -115,7 +119,8 @@ def edit_post(slug):
 def delete(slug):
     topic = Topics.find({"slug": slug})
     if not topic:
-        return redirect(url_for('.all_topics'))
+        abort(404)
+        return
 
     topic.delete()