more explicity payload err and msg

pkg/super_node/shared/types.go

@@ -20,7 +20,9 @@ package shared
 type RawChainData interface{}
 
 // The concrete type underneath StreamedIPLDs should not be a pointer
-type StreamedIPLDs interface{}
+type StreamedIPLDs interface {
+	Height() int64
+}
 
 type CIDsForIndexing interface{}
 
@@ -29,7 +31,9 @@ type CIDsForFetching interface{}
 type FetchedIPLDs interface{}
 
 // The concrete type underneath StreamedIPLDs should not be a pointer
-type ServerResponse interface{}
+type ServerResponse interface {
+	Height() int64
+}
 
 type Gap struct {
 	Start uint64