return 404 where it makes sense

app/routes/threads.py

@@ -1,5 +1,6 @@
 from flask import (
-    Blueprint, render_template, request, redirect, url_for, flash
+    Blueprint, render_template, request, redirect, url_for, flash,
+    abort,
     )
 from .users import login_required, mod_only, get_active_user, is_logged_in
 from ..db import db
@@ -32,7 +33,8 @@ def thread(slug):
     POSTS_PER_PAGE = 10
     thread = Threads.find({"slug": slug})
     if not thread:
-        return redirect(url_for('topics.all_topics'))
+        abort(404)
+        return
 
     post_count = Posts.count({"thread_id": thread.id})
     page_count = max(math.ceil(post_count / POSTS_PER_PAGE), 1)
@@ -86,7 +88,8 @@ def thread(slug):
 def reply(slug):
     thread = Threads.find({"slug": slug})
     if not thread:
-        return redirect(url_for('topics.all_topics'))
+        abort(404)
+        return
     user = get_active_user()
     if user.is_guest():
         return redirect(url_for('.thread', slug=slug))
@@ -148,7 +151,8 @@ def lock(slug):
     user = get_active_user()
     thread = Threads.find({'slug': slug})
     if not thread:
-        return redirect(url_for('topics.all_topics'))
+        abort(404)
+        return
     if not ((thread.user_id == user.id) or user.is_mod()):
         return redirect(url_for('.thread', slug=slug))
     target_op = request.form.get('target_op')
@@ -165,7 +169,8 @@ def sticky(slug):
     user = get_active_user()
     thread = Threads.find({'slug': slug})
     if not thread:
-        return redirect(url_for('topics.all_topics'))
+        abort(404)
+        return
     if not ((thread.user_id == user.id) or user.is_mod()):
         return redirect(url_for('.thread', slug=slug))
     target_op = request.form.get('target_op')