parentDirIsObject() to return quickly with inexistant parent (#11204)

Rewrite parentIsObject() function. Currently if a client uploads
a/b/c/d, we always check if c, b, a are actual objects or not.

The new code will check with the reverse order and quickly quit if 
the segment doesn't exist.

So if a, b, c in 'a/b/c' does not exist in the first place, then returns
false quickly.

cmd/storage-rest-client.go

@@ -82,6 +82,8 @@ func toStorageErr(err error) error {
 		return errFileNameTooLong
 	case errFileAccessDenied.Error():
 		return errFileAccessDenied
+	case errPathNotFound.Error():
+		return errPathNotFound
 	case errIsNotRegular.Error():
 		return errIsNotRegular
 	case errVolumeNotEmpty.Error():