Don't blurp on many clicks to clear alerts.

misago/apps/alerts.py

@@ -84,8 +84,12 @@ def clear_recent(request):
     if not request.is_ajax() or not request.method == 'POST':
         return error404(request)
 
-    del request.session['recent_alerts']
+    try:
+        del request.session['recent_alerts']
+    except KeyError:
+        pass
+
     response_html = render_to_string('alerts/cleared.html',
                                      context_instance=RequestContext(request))
     return json_response(request,
-                         json={'html': response_html})
+                         json={'html': response_html})