Merge pull request #5700 from ipfs/fix/is-dir-err

coreapi: fix errisdir

core/coreapi/interface/errors.go

@@ -3,6 +3,6 @@ package iface
 import "errors"
 
 var (
-	ErrIsDir   = errors.New("object is a directory")
+	ErrIsDir   = errors.New("this dag node is a directory")
 	ErrOffline = errors.New("this action must be run in online mode, try running 'ipfs daemon' first")
 )