move NotMutableError from dirnode.py into interfaces.py

src/allmydata/dirnode.py

@@ -7,7 +7,7 @@ from foolscap import Referenceable
 from allmydata import uri
 from allmydata.interfaces import RIVirtualDriveServer, \
      IDirectoryNode, IFileNode, IFileURI, IDirnodeURI, IURI, \
-     BadWriteEnablerError
+     BadWriteEnablerError, NotMutableError
 from allmydata.util import bencode, idlib, hashutil, fileutil
 from allmydata.Crypto.Cipher import AES
 
@@ -116,10 +116,6 @@ class VirtualDriveServer(service.MultiService, Referenceable):
 # whereas ImmutableDirectoryNodes and their support mechanisms live on the
 # client side
 
-class NotMutableError(Exception):
-    pass
-
-
 def create_directory_node(client, diruri):
     u = IURI(diruri)
     assert IDirnodeURI.providedBy(u)

src/allmydata/interfaces.py

@@ -1204,6 +1204,9 @@ class NotCapableError(Exception):
 class BadWriteEnablerError(Exception):
     pass
 
+class NotMutableError(Exception):
+    pass
+
 
 class RIControlClient(RemoteInterface):