Move add_child out of exception clause.
Move add_child out of exception clause.

It just makes it so that things don't double-raise
exceptions, which is especially painful in Python2.

Signed-off-by: Chris Lalancette <281cd07d7578d97c83271fbbf2faddb83ab3791c@gmail.com>

clalancette/pycdlib,clalancette/pyiso