make sure load_service_before_request handles 404s

if it 404s, because the service id doesn't exist, then it should die
gracefully (showing a 404 error page), rather than what it currently
does, which is die kicking and screaming with a 500

app/__init__.py

@@ -404,10 +404,23 @@ def load_service_before_request():
     if '/static/' in request.url:
         _request_ctx_stack.top.service = None
         return
-    service_id = request.view_args.get('service_id', session.get('service_id')) if request.view_args \
-        else session.get('service_id')
     if _request_ctx_stack.top is not None:
-        _request_ctx_stack.top.service = service_api_client.get_service(service_id)['data'] if service_id else None
+        _request_ctx_stack.top.service = None
+
+        if request.view_args:
+            service_id = request.view_args.get('service_id', session.get('service_id'))
+        else:
+            service_id = session.get('service_id')
+
+        if service_id:
+            try:
+                _request_ctx_stack.top.service = service_api_client.get_service(service_id)['data']
+            except HTTPError as exc:
+                # if service id isn't real, then 404 rather than 500ing later because we expect service to be set
+                if exc.status_code == 404:
+                    abort(404)
+                else:
+                    raise
 
 
 def save_service_after_request(response):

tests/app/main/test_errorhandlers.py

@@ -1,4 +1,6 @@
+from flask import Response
 from bs4 import BeautifulSoup
+from notifications_python_client.errors import HTTPError
 
 
 def test_bad_url_returns_page_not_found(client):
@@ -6,3 +8,16 @@ def test_bad_url_returns_page_not_found(client):
     assert response.status_code == 404
     page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
     assert page.h1.string.strip() == 'Page could not be found'
+
+
+def test_load_service_before_request_handles_404(client_request, mocker):
+    exc = HTTPError(Response(status=404), 'Not found')
+    get_service = mocker.patch('app.service_api_client.get_service', side_effect=exc)
+
+    client_request.get(
+        'main.service_dashboard',
+        service_id='00000000-0000-0000-0000-000000000000',
+        _expected_status=404
+    )
+
+    get_service.assert_called_once_with('00000000-0000-0000-0000-000000000000')