Products/TemporaryFolder/mount.py (its only client is
  Products/TemporaryFolder/TemporaryFolder.py).

- Moved exception 'MountedStorageError' from 'ZODB.POSExceptions'
  to 'Products.TemporaryFolder.mount' (now its only client).

- Removed spurious import-time dependencies from
  Products/ZODBMountPoint/MountedObject.py.