Show the user when there is no compatible video

[wmww]

Fixes #12.

Feel free to ignore/close if the project is no longer maintained. I'm considering using this as a base for my own open source casting project, so I figured I'd fix a bug just to get a feel of the code.

[hnryjms]

Aah, this looks great! Sorry I've been AWOL lately, but I'm back and gonna start picking up the maintenance here.

I'll take a deeper look at the code and try running it on my machine today, and go from there :)

[hnryjms]

Hey @wmww --

Looking at #14 it seems like there's some future scope of supporting multiple <video /> elements on the screen. I'm wondering if it would make more sense for us to update the Event types to be an array of the <video /> elements, and the popup code displays the error message based on the array being empty.

For now, we could still just auto-select the first video, but maybe down the road we are set up to build some type of picker inside the popup to choose between the multi-video page.