store: catch when file does not exist

store.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"log"
 	"os"
@@ -26,11 +27,17 @@ type Store string
 func (s Store) Set(key string, value string) {
 	err := os.WriteFile(path.Join(string(s), key), []byte(value), 0600)
 	if err != nil {
-		log.Println(err)
+		log.Println(fmt.Errorf("failed to set %q: %s", key, err))
 	}
 }
 
 func (s Store) Get(key string) (string, error) {
+	keyPath := path.Join(string(s), key)
+	_, err := os.Stat(keyPath)
+	if errors.Is(err, os.ErrNotExist) {
+		return "", os.ErrNotExist
+	}
+
 	data, err := os.ReadFile(path.Join(string(s), key))
 	if err != nil {
 		return "", nil