Treat responses with status >= 500 as invalid responses

Do not expect to be able to parse valid session data out of responses
with 5xx status codes - eg. gateway errors, internal server errors.

h/static/scripts/session.js

@@ -155,7 +155,7 @@ function session($http, $resource, $rootScope, flash, raven, settings) {
   };
 
   function process(data, headersGetter, status) {
-    if (status < 200) {
+    if (status < 200 || status >= 500) {
       return;
     }
 

h/static/scripts/test/session-test.js

@@ -159,10 +159,20 @@ describe('h:session', function () {
       $httpBackend.flush();
     });
 
-    it('should tolerate failed requests', function () {
-      $httpBackend.expectGET(url).respond(-1, null);
-      session.load();
-      $httpBackend.flush();
+    var failedRequestCases = [{
+      status: -1,
+      body: null,
+    },{
+      status: 504,
+      body: 'Gateway Timeout',
+    }];
+
+    failedRequestCases.forEach(function (testCase) {
+      it('should tolerate failed requests', function () {
+        $httpBackend.expectGET(url).respond(testCase.status, testCase.body);
+        session.load();
+        $httpBackend.flush();
+      });
     });
   });