Fix byte compile warnings

formatting error is raised if `nil` is quoted.
I got following error with original code when byte-compile.

```
In tgt-find-project-file-in-dirs:
toggle-test.el:112:37:Warning: `error' called with 1 args to fill 0 format
    field(s)
toggle-test.el:116:49:Warning: `error' called with 1 args to fill 0 format
    field(s)
```