Avoid behaviour change by raising Http404 for invalid page numbers

An earlier version of this branch would always display the first page in this
case. For compatibility we now simply return 404.

haystack/views.py

@@ -99,7 +99,7 @@ def build_page(self):
         try:
             page_no = int(self.request.GET.get('page', 1))
         except (TypeError, ValueError):
-            page_no = 1
+            raise Http404
 
         start_offset = (page_no - 1) * self.results_per_page
         self.results[start_offset:start_offset + self.results_per_page]