build-image: Exit in case of an unknown dist

When using the spark builder setup don't carry on if the dist
is unknown.

build-image

@@ -132,6 +132,7 @@ setup_laniakea_spark_build() {
         distro=amber-phone
       ;;
       *) echo "Unknown dist"
+        exit 1
       ;;
   esac
   apt-get update