[KV] Consider throwing BackupAgent a failure

And not a success as it used to be.

Bug: 111051813
Bug: 110082831
Test: atest KeyValueBackupTaskTest
Test: 1. BackupApp throwing in onBackup.
      2. adb shell bmgr backupnow com.google.android.apps.backupapp
      3. Verify logs and that it threw and we did not save backup data for it.
Test: 1. BackupApp not throwing.
      2. adb shell bmgr backupnow com.google.android.apps.backupapp
      3. Verify logs and that data was sent to transport.

Change-Id: Idb7fe298f64786668989c30cdce53355aeef7277