Error message associated with not having run `cargo run --bin updater` could be clearer

Running

```
cargo run --bin game
```

when the import is out of date results in a difficult-to-interpret stream of messages on the CLI and this message on the GUI:


A message saying something like:

> Imported data may be out of date, try running
> `cargo run --bin updater`

Would solve the issue I think. Details of the output from the command line below in case of use/interest.

```
cargo run --bin game
Finished dev [unoptimized + debuginfo] target(s) in 0.16s
Running `target/debug/game`
setup widgetry...
[2021-04-19T22:09:55Z INFO winit::platform_impl::platform::x11::window] Guessed window scale factor: 1
load textures...
[2021-04-19T22:09:55Z INFO widgetry::backend_glow] img_size: 896x896px (802816 px), sprite_size: 64x64px, sprites: 14x14 (196 sprites)
load textures took 0.1397s
setup app...
[2021-04-19T22:09:55Z INFO map_gui::render::map] static DrawMap consumes 0 MB on the GPU
setup app took 0.0008s
setup widgetry... plus 0.0962s
setup widgetry took 0.2367s

- setup widgetry took 0.2367s
- load textures took 0.1397s
- setup app took 0.0008s
- ... plus 0.0962s
load data/system/us/seattle/maps/montlake.bin...
Read data/system/us/seattle/maps/montlake.bin (4)... 0.6804s
load data/system/us/seattle/maps/montlake.bin... plus 0.0687s
load data/system/us/seattle/maps/montlake.bin took 0.7491s

- load data/system/us/seattle/maps/montlake.bin took 0.7491s
- Read data/system/us/seattle/maps/montlake.bin (4)... 0.6804s
- ... plus 0.0687s
[2021-04-19T22:09:56Z WARN widgetry::app_state] A state requested DrawBaselayer::PreviousState, but it's the only state on the stack!
```