Mark _init as global in pyafs_error

Because _init is later assigned to in the function, new enough versions of
cython (67bf84357feabe73612e2a9eef3e48a05e74d76f or so and later, apparently)
conclude that it's a local. Unfortunately, that means that the "if _init" is
referencing an undeclared variable. See also
http://codespeak.net/pipermail/cython-dev/2009-July/006396.html.

Signed-off-by: Alex Dehnert <adehnert@mit.edu>