__done__( [error[, results]] )
    Callback invoked to confirm that an asynchronous command has finished
    executing.

    This callback should only be called *once* per command.

    In environments supporting async/await, top-level await commands are not
    required to invoke this callback.

    Parameters
    ----------
    error: Error|null (optional)
        Error, if an error occurred during execution, and `null`, if no error
        occurred.

    results: any (optional)
        Command results.

    Examples
    --------
    > function onTimeout() { __done__( null, 'foo' ) };
    > var res = setTimeout( onTimeout, 1000 )
    [ <number>, 'foo' ]