add a little bit more verbosity to the error

License: MIT
Signed-off-by: Jeromy <why@ipfs.io>

merkledag/merkledag.go

@@ -89,7 +89,7 @@ func (n *dagService) Get(ctx context.Context, k key.Key) (*Node, error) {
 	res, err := DecodeProtobuf(b.Data())
 	if err != nil {
 		if strings.Contains(err.Error(), "Unmarshal failed") {
-			return nil, fmt.Errorf("%s was not a valid merkledag node", k)
+			return nil, fmt.Errorf("The block referred to by '%s' was not a valid merkledag node", k)
 		}
 		return nil, fmt.Errorf("Failed to decode Protocol Buffers: %v", err)
 	}