src/vs/workbench/api/common/extHostLanguageFeatures.ts/RenameAdapter/_asMessage
class RenameAdapter:	private static _asMessage(err: any): string | undefined {
		if (typeof err === 'string') {
			return err;
		} else if (err instanceof Error && typeof err.message === 'string') {
			return err.message;
		} else {
			return undefined;
		}
	}