modemmanager-next/samsung: Check error when activation fails

Port code from old ModemManager to issue an %IER command when %IPDPACT
returns a failure, and pass the relevant error up the stack.

BUG=chromium-os:30656
TEST=With an AT&T SIM, try to connect with an bogus APN.
"mmcli -m 0 --simple-connect='apn=bogus'".
mmcli should return with error #MobileEquipment.Gprs.ServiceOptionNotSubscribed.

Change-Id: I8abb70185d488f32574f8abff2e5f7905ea4e935