encoding: add TODOs for producing better error messages with handling of Any

Change-Id: Iaab531d8ae454715338bf079a2749afeedc978f9
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/175577
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/encoding/textpb/encode.go b/encoding/textpb/encode.go
index bce34b7..78ed3a2 100644
--- a/encoding/textpb/encode.go
+++ b/encoding/textpb/encode.go
@@ -362,6 +362,9 @@
 	}
 	em := emt.New().Interface()
 	// TODO: Need to set types registry in binary unmarshaling.
+	// TODO: If binary unmarshaling returns required not set error, need to
+	// return another required not set error that contains both the path to this
+	// field and the path inside the embedded message.
 	err = proto.UnmarshalOptions{
 		AllowPartial: o.AllowPartial,
 	}.Unmarshal(value.Bytes(), em)