#31 Automatically link to the previous day's image

[Andrew GJ Fung]

Personally, I prefer not to see an error message that
today's strip can't be downloaded. Since I sometimes
miss a day or two, showing the previous day's image is
acceptable to me. I suppose ideally we could show the
image *and* print an error.

I admit to not looking thoroughly to ensure that this
code patch "fits" properly, but as far as I can tell,
it just does a minor tweak with no adverse side
effects, and then slides back into the "regular" code.

[Andrew GJ Fung]

Patch to automatically symlink to previous day's image if today's image has an error.