Raise verbose errors when failing to save a user

[jeancochrane]

If a user signs up but something is wrong with the database such that an IntegrityError gets raised, the code will currently suppress the error and show a generic error to the user:

django-councilmatic-notifications/notifications/views.py

Lines 139 to 154 in 0aca66b

	try:
	user = form.save()
	redirect = reverse('index')
	if request.GET.get('next'):
	redirect = request.GET['next']
	host = '{0}://{1}'.format(request.scheme, request.get_host())
	send_signup_email(user, host, redirect)
	messages.add_message(request, messages.INFO, 'Check your email for a link to confirm your account!')
	return HttpResponseRedirect(redirect)
	except IntegrityError:
	response = HttpResponse('Not able to save form.')
	response.status_code = 500
	return response

This happened recently when the primary key sequence for the auth_user table got out of sync, causing an IntegrityError to get raised by form.save().

We should make sure that we log this error to Sentry so that we can diagnose the error without having to interrupt the running application.