Downgrade exception to warning for provider API call

When we send an HTTP request to our SMS providers, there is a
chance we get a 5xx status code back from them. Currently we log this as
two different exception level logs.

If a provider has a funny few minutes, we could end up with
hundreds of exceptions thrown and pagerduty waking someone up in the
middle of the night. These problems tend to pretty quickly fix
themselves as we balance traffic from one SMS to the other SMS provider
within 5 minutes.

By downgrading both exceptions to warning in the case of a
`SmsClientResponseException`, we will reduce the change of waking us up
in the middle of the night for no reason.

If the error is not a `SmsClientResponseException`, then we will still
log at the exception level as before as this is more unexpected and we
may want to be alerted sooner.

What we still want to happen though is that let's say both SMS providers
went down at the same time for 1 hour. We don't want our tasks to just
sit there, retrying every 5 minutes for the whole time without us being
aware (so we can at least raise a statuspage update). Luckily we will
still be alerted because our smoke tests will fail after 10 minutes and
raise a p1:
https://github.com/alphagov/notifications-functional-tests/blob/master/tests/functional/staging_and_prod/notify_api/test_notify_api_sms.py#L21

app/celery/provider_tasks.py

@@ -6,6 +6,7 @@ from app import notify_celery
 from app.config import QueueNames
 from app.clients.email import EmailClientNonRetryableException
 from app.clients.email.aws_ses import AwsSesClientThrottlingSendRateException
+from app.clients.sms import SmsClientResponseException
 from app.dao import notifications_dao
 from app.dao.notifications_dao import update_notification_status_by_id
 from app.delivery import send_to_providers
@@ -22,11 +23,17 @@ def deliver_sms(self, notification_id):
         if not notification:
             raise NoResultFound()
         send_to_providers.send_sms_to_provider(notification)
-    except Exception:
-        try:
+    except Exception as e:
+        if isinstance(e, SmsClientResponseException):
+            current_app.logger.warning(
+                "SMS notification delivery for id: {} failed".format(notification_id)
+            )
+        else:
             current_app.logger.exception(
                 "SMS notification delivery for id: {} failed".format(notification_id)
             )
+
+        try:
             if self.request.retries == 0:
                 self.retry(queue=QueueNames.RETRY, countdown=0)
             else: