build/azure-pipelines/common/publish-webview.ts/doesBlobExist
async function doesBlobExist(blobService: azure.BlobService, container: string, blobName: string): Promise<boolean | undefined> {
	const existsResult = await new Promise<azure.BlobService.BlobResult>((c, e) => blobService.doesBlobExist(container, blobName, (err, r) => err ? e(err) : c(r)));
	return existsResult.exists;
}