Show errors thrown before the Streamlit server is launched in the WebWorker on the mountable app

[whitphx]

Currently, if some error occurs before the Streamlit server launch, the screen stays "Loading...".
It should indicate the error.

[whitphx]

It is displayed on a toast now.

stlite/packages/kernel/src/worker.ts

Lines 247 to 253 in 762c9a6

	ctx.postMessage({
	type: "event:error",
	data: {
	error,
	},
	});
	throw error;