add assertions for blind grep

blind grep is error-prone.
Add assertions for errors we can not handle

Change-Id: Ibe19085545ecc848498506e8b8ee14e71825b273
diff --git a/exercises/aggregates.sh b/exercises/aggregates.sh
index e5fc7de..6cc81ae 100755
--- a/exercises/aggregates.sh
+++ b/exercises/aggregates.sh
@@ -66,7 +66,10 @@
 exit_if_aggregate_present $AGGREGATE_NAME
 
 AGGREGATE_ID=$(nova aggregate-create $AGGREGATE_NAME $AGGREGATE_A_ZONE | grep " $AGGREGATE_NAME " | get_field 1)
+die_if_not_set $LINENO AGGREGATE_ID "Failure creating AGGREGATE_ID for $AGGREGATE_NAME $AGGREGATE_A_ZONE"
+
 AGGREGATE2_ID=$(nova aggregate-create $AGGREGATE2_NAME $AGGREGATE_A_ZONE | grep " $AGGREGATE2_NAME " | get_field 1)
+die_if_not_set $LINENO AGGREGATE2_ID "Fail creating AGGREGATE2_ID for $AGGREGATE2_NAME $AGGREGATE_A_ZONE"
 
 # check aggregate created
 nova aggregate-list | grep -q " $AGGREGATE_NAME " || die $LINENO "Aggregate $AGGREGATE_NAME not created"