WebDriver: Spec wants "dispatch actions inner" to throw exceptions

[AtkinsSJ]

See spec change at w3c/webdriver#1861

I had a quick go at implementing this but our code implements the waiting step by queueing tasks, which is incompatible with returning an exception in this way. So leaving it for someone who understands this code better. cc: @trflynn89

[trflynn89]

Oh that should be pretty straight forward - we have a hook to send exceptions back to the client at any point in the async chain. Something like:

// 1. If browsing context is no longer open, return error with error code no such window.
if (m_browsing_context->has_navigable_been_destroyed()) {
    m_on_complete->function()(WebDriver::Error::from_code(WebDriver::ErrorCode::NoSuchWindow, "Window has been closed"sv));
    return;
}

// 2. Let tick duration be the result of computing the tick duration with argument tick actions.
auto tick_duration = compute_tick_duration(tick_actions);

(In an actual PR, note that we have a helper for the "browsing context is open" check in WebDriverConnection.cpp that we should probably move some place public and use here).