Better error handling around ffprobe call

[zfleeman]

There are a lot of assumptions that I make about the metadata available in the video file. I've fixed the "display_aspect_ratio" problem in this commit, but I should check for more issues like:

• "display_aspect_ratio" is not always tagged on a video file
• ["format"]["duration"]
• ["streams"]
• No audio stream video files break in the self.run() function