@icon support as of #1096 works consistently and well, but whenver there is an error (e.g. downloading the URL @icon references) it is a completely fatal error -- a modal alert() blocks, and the script does not install.
Instead, the error should be logged to the Error Console, and the script should continue to install, simply without an icon.
Test case: https://gist.github.com/1072748
The first version, a nonexistent host, works. The second, pointing at localhost (where I do have a web server, but not an "error.png" in the root) fails.
Ignore failed icon downloads.