python: Set status in the class definitions

Technically, this is a superfluous change, as the self.status variable
currently gets set in NotmuchErrors's __new__ function. However, in the
long run I would like to get rid of the weird __new__ implementation which
might be somewhat confusing for users (NotmuchError(status) returns a
different class, e.g. OutOfMemoryError)

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>