Fix #161

22333a3221eb5e7f10dd1770fb597d114f27a2c7 added a second reference to `PylibMCExc_Error` without a corresponding `Py_INCREF` call. This is caught by an assertion enabled in debug builds of Python, which causes a core dump as described in #161.

I'm not sure if this is the best place to add this `Py_INCREF` call, but it definitely has to go *somewhere*.