Improve error message and UX when user accidentally unshared a video stream

[LukasKalbertodt]

By @ziegenberg here.

If you stop sharing the video or screen then the following error message is displayed: "Video stream unexpectedly ended! Did a camera disconnect or did you accidentally unshare a video source?" If this was accidental, the user probably wants to undo this. We should probably let the user know, that the browser window needs to be reloaded to choose again the video source. Otherwise, the user might complain that the select sources button have "just vanished".