FuncGroup: move return type test
FuncGroup: move return type test

If a FuncGroup function returns something else than a Submittable or
Group (e.g., None; programming error), raise a TypeError before trying
to call propagate_attribute.

Signed-off-by: Yves Bastide <3b1fe340dba76bf37270abad774f327f50b5e1d8@botify.com>

botify-labs/simpleflow,botify-labs/simpleflow