When some file is missing the error message doesn't mention the file name

Resources were missing and this was the message:
```
$ abstreet-game
[2021-11-08T17:19:04Z WARN map_gui::options] Couldn't restore settings, so using defaults. No such file or directory (os error 2)
setup widgetry...
[2021-11-08T17:19:04Z INFO winit::platform_impl::platform::x11::window] Guessed window scale factor: 1
load textures...
[2021-11-08T17:19:04Z INFO widgetry::backend_glow] img_size: 896x896px (802816 px), sprite_size: 64x64px, sprites: 14x14 (196 sprites)
load textures took 0.0628s
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: IoError(Os { code: 2, kind: NotFound, message: "No such file or directory" })', /disk-samsung/freebsd-ports/games/abstreet/work/abstreet-0.2.64/widgetry/src/runner.rs:316:43
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
setup widgetry... plus 0.1795s
setup widgetry took 0.2423s
[2021-11-08T17:19:04Z INFO abstutil::time] - setup widgetry took 0.2423s
[2021-11-08T17:19:04Z INFO abstutil::time] - load textures took 0.0628s
[2021-11-08T17:19:04Z INFO abstutil::time] - ... plus 0.1795s
[2021-11-08T17:19:04Z ERROR abstutil::time]
[2021-11-08T17:19:04Z ERROR abstutil::time]
[2021-11-08T17:19:04Z ERROR abstutil::time]
[2021-11-08T17:19:04Z ERROR abstutil::time]
[2021-11-08T17:19:04Z ERROR abstutil::time]
[2021-11-08T17:19:04Z ERROR abstutil::time] !!! The program panicked, look above for the stack trace !!!
```

In fact. ```icon.png``` was missing.