Handle wgsl errors in the game of life example

[kornelski]

Currently copypasting the example into a new project without also copying "shaders/game_of_life.wgsl" gives an unhelpful blank screen.
This change makes it panic instead. I think nicer error handling is outside scope of the example, and this is good enough to point out that the shader code is missing.

[kornelski]

This affects other examples, so if this approach is okay, I can repeat it for other examples too.

[IceSentry]

This affects other examples

IIRC game_of_life is the only example that does that kind of thing, I'm not sure which other example you are referring to.

[kornelski]

I mean there's a pattern of examples/foo.rs using assets/shaders/foo.rs

https://github.com/bevyengine/bevy/blob/main/examples/shader/array_texture.rs

bevy/examples/shader/array_texture.rs

Line 92 in 09fb6b4

"shaders/array_texture.wgsl".into()

https://github.com/bevyengine/bevy/blob/main/assets/shaders/array_texture.wgsl

and the example code run outside of bevy's directory structure fails to load the files it depends on, and displays a blank screen:

ERROR bevy_asset::server: Path not found: /tmp/assets/shaders/array_texture.wgsl

I'm proposing to move these paths to a const near the top of the file, so that users will more likely notice the dependency.

[alice-i-cecile]

I'd be happy to see this done more broadly in some form. The current PR is an improvement though so I'll merge it on Monday unless you tell me not to.

[rparrett]

I think moving the shader path into a constant is a great idea.

The actual error produced here feels confusing to me, which is unfortunate.

thread '<unnamed>' panicked at examples/shader/compute_shader_game_of_life.rs:233:25:
Initializing assets/shaders/game_of_life.wgsl:
Pipeline could not be compiled because the following shader is not loaded yet: AssetId<bevy_render::render_resource::shader::Shader>{ index: 0, generation: 0}
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Encountered a panic in system `bevy_render::renderer::render_system`!

Specifically, the not loaded *yet* part. I wonder if that could be improved (not in this PR), or if there are scenarios where that language actually makes sense?