python: rework creating of Subclasses

Add some smart magic, so that when we invoke a
NotmuchError(STATUSVALUE), a nicely derived subclass is created, e.g. a
OutOfMemoryError. This way users can easily distinguish between error
types, while still catching NotmuchError.

I have tested this, and hope it works for others too.

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