Infinite loop if target directory vanishes.

[adeutscher]

Steps to reproduce:

1. Run Updog on a directory.
2. While Updog is running, remove/rename the directory.
3. Visit Updog in a browser. The browser will be told it isn't redirecting properly.

[adeutscher]

On a similar note, there'll be a 500 if the directory was replaced with a file. This seems less of a problem than the many requests that will come in before the browser (hopefully) catches on.