test: Catch possible errors in the yamlfmt test

We should not use -o errfail because that will not give diagnostic
ouput. But we should catch errors from as many places as possible.

test/test-yamlfmt.sh

@@ -2,6 +2,7 @@
 # check for any yaml files that aren't properly formatted
 
 echo running "$0"
+set -o pipefail
 
 #ROOT="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && cd .. && pwd )" # dir!
 ROOT=$(dirname "${BASH_SOURCE}")/..
@@ -14,7 +15,8 @@ if [ -z "$LINTER" ]; then
 fi
 
 find_files() {
- git ls-files | grep '\.yaml$'
+ git ls-files | grep '\.yaml$' \
+ || fail_test "Could not find yaml files via git ls-files"
 }
 
 bad_files=$(