Milo: Add themed page for errors

milo/appengine/buildbot/builder.go

Index: milo/appengine/buildbot/builder.go
diff --git a/milo/appengine/buildbot/builder.go b/milo/appengine/buildbot/builder.go
index 655b5bc013463cc6cb7bf943217359ff7db00574..1231693597466c8f37c7716e72faef2be4c67316 100644
--- a/milo/appengine/buildbot/builder.go
+++ b/milo/appengine/buildbot/builder.go
@@ -78,10 +78,15 @@ func getBuilds(
 }
 
 var errMasterNotFound = miloerror.Error{
-	Message: "Master not found",
+	Message: "Either the request resource was not found or you are not authorized",
 	Code:    http.StatusNotFound,
 }
 
+var errNotAuth = miloerror.Error{
+	Message: "You are not authenticated, try logging in",
+	Code:    http.StatusUnauthorized,
+}
+
 // builderImpl is the implementation for getting a milo builder page from buildbot.
 // This gets:
 // * Current Builds from querying the master json from the datastore.