change pylibmc MemcachedError exception import

Wouldn't it be more Pythonic and lessen the risk of breakage due to future changes in pylibmc to do the import without referencing the protected namespace.

http://sendapatch.se/projects/pylibmc/misc.html#exceptions

```
python tests/tests.py
Creating test database for alias 'default'...
.....................................................................
----------------------------------------------------------------------
Ran 69 tests in 22.331s

OK
Destroying test database for alias 'default'...
```