Only link to message status if message has failed

This retores the behaviour to as it was before
https://github.com/alphagov/notifications-admin/pull/2962
which inadvertently started linking to the guidance for messages that
were delivered or in sending.

tests/app/main/test_formatters.py

@@ -0,0 +1,35 @@
+from functools import partial
+
+import pytest
+from flask import url_for
+
+from app import format_notification_status_as_url
+
+
+@pytest.mark.parametrize('status, notification_type, expected', (
+    # Successful statuses aren’t linked
+    ('created', 'email', lambda: None),
+    ('sending', 'email', lambda: None),
+    ('delivered', 'email', lambda: None),
+    # Failures are linked to the channel-specific page
+    ('temporary-failure', 'email', partial(url_for, 'main.message_status', _anchor='email-statuses')),
+    ('permanent-failure', 'email', partial(url_for, 'main.message_status', _anchor='email-statuses')),
+    ('technical-failure', 'email', partial(url_for, 'main.message_status', _anchor='email-statuses')),
+    ('temporary-failure', 'sms', partial(url_for, 'main.message_status', _anchor='sms-statuses')),
+    ('permanent-failure', 'sms', partial(url_for, 'main.message_status', _anchor='sms-statuses')),
+    ('technical-failure', 'sms', partial(url_for, 'main.message_status', _anchor='sms-statuses')),
+    # Letter statuses are never linked
+    ('technical-failure', 'letter', lambda: None),
+    ('cancelled', 'letter', lambda: None),
+    ('accepted', 'letter', lambda: None),
+    ('received', 'letter', lambda: None),
+))
+def test_format_notification_status_as_url(
+    client,
+    status,
+    notification_type,
+    expected,
+):
+    assert format_notification_status_as_url(
+        status, notification_type
+    ) == expected()