Add quotes around string in error message

enumer.go

@@ -10,7 +10,7 @@ func %[1]sString(s string) (%[1]s, error) {
 	if val, ok := _%[1]sNameToValueMap[s]; ok {
 		return val, nil
 	}
-	return 0, fmt.Errorf("%%s does not belong to %[1]s values", s)
+	return 0, fmt.Errorf("'%%s' does not belong to %[1]s values", s)
 }
 `