Bad YAML = showstopper

Fixes #154

abra

@@ -478,6 +478,16 @@ require_docker_version() {
   done
 }
 
+require_valid_json() {
+  require_jq
+  $JQ "$1" > /dev/null || error "Invalid JSON '$1'"
+}
+
+require_valid_yaml() {
+  require_yq
+  $YQ e "$1" > /dev/null || error "Invalid YAML '$1'"
+}
+
 ###### Download and update data
 
 require_apps_json() {
@@ -702,6 +712,7 @@ output_version_summary() {
 
   IFS=':' read -ra COMPOSE_FILES <<< "$COMPOSE_FILE"
   for COMPOSE in "${COMPOSE_FILES[@]}"; do
+    require_valid_yaml "$APP_DIR/$COMPOSE"
     SERVICES=$($YQ e '.services | keys | .[]' "${APP_DIR}/${COMPOSE}")
 
     for SERVICE in $SERVICES; do