brctl doesn't give an exit code indicating success or failure.
# echo "alias bridge off" >> /etc/modules.conf ; brctl addbr br0 ; echo $?
br_add_bridge: Package not installed
Ideally, there would at least be a 'test if bridging support is available'
command; 'brctl show > /dev/null' would suffice, if it was going to return
failure from that.