Give reason for early exit in shell script

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: fanninpm

scripts/whats_left_builtin_items.sh

@@ -6,6 +6,7 @@ mkdir -p ../_data/whats_left
 
 # exit violently if the temp file does not exist
 if [ ! -f ../_data/whats_left.temp ]; then
+    echo "error: input file ../_data/whats_left.temp not found" >&2
     exit 1
 fi