src/vs/workbench/services/extensions/common/extensions.ts/checkProposedApiEnabled
export function checkProposedApiEnabled(extension: IExtensionDescription, proposal: ApiProposalName): void {
	if (!isProposedApiEnabled(extension, proposal)) {
		throw new Error(`Extension '${extension.identifier.value}' CANNOT use API proposal: ${proposal}.\nIts package.json#enabledApiProposals-property declares: ${extension.enabledApiProposals?.join(', ') ?? '[]'} but NOT ${proposal}.\n The missing proposal MUST be added and you must start in extension development mode or use the following command line switch: --enable-proposed-api ${extension.identifier.value}`);
	}
}