casting attempt feedback?

[tomByrer]

I clicked on [Request Cast] in demo app, but there was no UI nor console feedback that there was no Cast devise set up. (I clicked knowing it would fail).

Thanks for building this; might end up using for a video education platform.

[luwes]

thanks for the quick feedback, I'll add this as a todo!

normally one would show / hide the cast button with the castchange event like we do in Media Chrome https://github.com/muxinc/media-chrome/pull/220