Fix regression

Fix a regression.

An error in the [RTD docs build](https://readthedocs.org/builds/pillow/1728218/):
```
/var/build/user_builds/pillow/checkouts/latest/docs/reference/ImageCms.rst:11: WARNING: autodoc: failed to import module u'PIL.ImageCms'; the following exception was raised:
Traceback (most recent call last):
  File "/home/docs/local/lib/python2.7/site-packages/sphinx/ext/autodoc.py", line 335, in import_object
    __import__(self.modname)
  File "/home/docs/local/lib/python2.7/site-packages/PIL/ImageCms.py", line 93, in <module>
    from _util import import_err
ImportError: cannot import name import_err
```

`import_err()` was introduced on 4th April in _util.py and ImageCms.py by https://github.com/python-pillow/Pillow/commit/90bbd9ff3ecb4b69a04b37ea40efda4cb8e82b00

and renamed to `deferred_error()` in the same files on 9th April by https://github.com/python-pillow/Pillow/commit/b27ef7646855d4e27d8e3dca528fd2e847756ac8.

Both of these are shown as in being in master, 2.6.0, 2.5.0, etc., but for some reason there's now `deferred_error()` in _util.py and `import_err()` in ImageCms.py.

Blame shows only the former and not the later rename in ImageCms.py:
https://github.com/python-pillow/Pillow/blame/master/PIL/ImageCms.py#L93

Anyway, here's a fix.