Add additional error handling when video devices aren't able to be opened.

[colinbdclark]

On Windows, it sounds like it may not be possible to share camera devices among multiple applications, so it's easier to get into a situation where MovementOSC can't successfully open the MediaStream. We should improve our error handling and report back to the user when a camera stream can't be opened.

[colinbdclark]

@amb26 reports that on his WIndows 10 computer, the DevTools console reports an uncaught DOMException:

"Uncaught DOMException: Could not start video source"