Stop logging proxied 5XX Graphite errors

This adds what amounts to a dirty hack to the graphite-web proxy
view (i.e. it reaches into Django internals) to prevent Django from
doing a full error response log when proxying 5xx class errors from
graphite-web.  It does so by tricking `django.utils.log.log_response()`
into thinking that the response has already been logged (since our
view function has no other control over how its return values are
logged by Django).

The reason for the 5xx response codes are not bugs in our view code,
and does not need to trigger site admin e-mails.

python/nav/web/graphite/views.py

@@ -73,6 +73,14 @@ def index(request, uri):
     else:
         response = HttpResponse(output, content_type=content_type, status=status)
 
+    # Dirty hack to prevent logging of 5XX errors.  More specifically, this prevents
+    # a full Django error mail from being sent when Graphite is down or otherwise
+    # failing.  (I.e. we're just proxying the status code, there is no error in our
+    # application that needs to be mailed to anyone).
+    if status >= 500:
+        # Tricks Django into thinking this response has already been logged:
+        response._has_been_logged = True
+
     response['X-Where-Am-I'] = request.get_full_path()
     return response