WebGL and WebGPU backends should not panic if canvas context is unavailable

[kpreid]

The WebGL and WebGPU backends have code which expect()s success in obtaining a canvas:

wgpu/wgpu/src/backend/web.rs

Lines 956 to 959 in 94ce763

	let context: wasm_bindgen::JsValue = match canvas.get_context("webgpu") {
	Ok(Some(ctx)) => ctx.into(),
	_ => panic!("expected to get context from canvas"),
	};

wgpu/wgpu-hal/src/gles/web.rs

Lines 36 to 41 in 94ce763

	let webgl2_context = canvas
	.get_context_with_context_options("webgl2", &Self::create_context_options())
	.expect("Cannot create WebGL2 context")
	.and_then(|context| context.dyn_into::<web_sys::WebGl2RenderingContext>().ok())
	.expect("Cannot convert into WebGL2 context");

These will panic if the browser declines to provide a context, which may happen for reasons outside of the application programmer's control (browser does not support WebGPU or WebGL2, insufficient GPU memory, browser glitches) as well as ones which are (such as trying to use the same canvas for "2d" drawing). Therefore, the application may have reason to handle the error (if nothing else, to give the user an error message visible on the web page), which is best done by returning an Err rather than panicking — especially as Rust on wasm32 does not support unwinding, so every panic is fatal to the Rust module and can at best be handled from JavaScript.

This is not a solely theoretical scenario — I noticed the problem due to some Bevy Jam #2 games failing to start.

Platform
web, wgpu 0.13