Fix a regression in generate_upper_constraints.sh

Commit 041c1795db has an issue that
prevents building IPA when requirements are not available locally.

Change-Id: I93d5fdb599315922f35a3caaf311b14d8a74a858
Story: #2004776
Task: #28901

imagebuild/common/generate_upper_constraints.sh

@@ -62,6 +62,10 @@ copy_new_requirements_uc() {
         copy "/opt/stack/new/requirements/upper-constraints.txt" "${DESTINATION}"
     elif [ -e "/opt/stack/requirements" ]; then
         copy "/opt/stack/requirements/upper-constraints.txt" "${DESTINATION}"
+    else
+        log "No local requirements repository, will download upper-constraints"
+        # Allow the caller to handle the failure
+        return 1
     fi
 }