Properly report failure in runIptablesAlertCmd.

(cherry picked from commit 7647305c6b13d0e448b055c8af9c09b34af79f5c)

Bug: 37641280
Test: netd_{unit,integration}_test pass
Change-Id: Ic2b692efae14c4c9ca19972bdd812edce1c39bb3
Merged-In: I36ef121ae0cfaa16032289fa6f8b0341e1a9ca20
1 file changed