[BUG] Django error e-mail is sent to site admins when Graphite is not responding properly

[lunkwill42]

Describe the bug

NAV's /graphite view works as a proxy to the configured graphite-web server for authenticated users (since graphite-web provides no proper authorization scheme):

nav/python/nav/web/graphite/views.py

Lines 30 to 33 in e771213

	def index(request, uri):
	"""
	Proxies render requests to graphite-web, as configured in graphite.conf
	"""

The view proxies both response status codes and response bodies. There is, however, a drawback to the current implementation: Any 5XX type response code from graphite-web will trigger Django's log handler to send a full error message to the NAV site admins, even if there is no exception or error in NAV code (it simply triggers on the fact that the index() view function returns a HttpResponse object with status>=500).

We've seen cases where someone has built dashboards of NAV graphs, and if the dashboards refresh just as the graphite-web service is being restarted, hundreds of these e-mails are dispatched to the site admins, which is "inconvenient" (i.e. it is unnecessary noise!).

The sent email will often contain all request details, but no traceback, since there is no actual exception raised within the view code.

To Reproduce

1. Open a NAV graph (e.g. something like this )
2. Stop graphite-web (or replace it with a web service that always responds with e.g. 503 Service Unavailable).
3. Reload the graph
4. Observe e-mail received by site admin (only when running with DJANGO_DEBUG=False in nav.conf)

Expected behavior

5xx errors should be passed on to the client without error mails being sent to the site admins.

Environment (please complete the following information):

• NAV version installed: 5.12.0 (and earlier)