Milo: Handle missing / transient LogDog failures.

milo/appengine/logdog/http.go

Index: milo/appengine/logdog/http.go
diff --git a/milo/appengine/logdog/http.go b/milo/appengine/logdog/http.go
index 43190b9074cec535178ce247f70b67646da313e2..906d39bed388ff42ad3c01542267bd478d08d3aa 100644
--- a/milo/appengine/logdog/http.go
+++ b/milo/appengine/logdog/http.go
@@ -61,7 +61,25 @@ func (s *AnnotationStreamHandler) Render(c context.Context, req *http.Request, p
 
 	// Load the Milo annotation protobuf from the annotation stream.
 	if _, err := as.Fetch(c); err != nil {
-		return nil, err
+		switch errors.Unwrap(err) {
+		case coordinator.ErrNoSuchStream:
+			return nil, &miloerror.Error{
+				Message: "Stream does not exist",
+				Code:    http.StatusNotFound,
+			}
+
+		case coordinator.ErrNoAccess:
+			return nil, &miloerror.Error{
+				Message: "No access to stream",
+				Code:    http.StatusForbidden,
+			}
+
+		default:
+			return nil, &miloerror.Error{
+				Message: "Failed to load stream",
+				Code:    http.StatusInternalServerError,
+			}
+		}
 	}
 
 	// Convert the Milo Annotation protobuf to Milo objects.