Tools should check for HTTP failure when downloading

[tmo1]

I was running prefsCleaner.sh today, and got the following error:

/home/username/.mozilla/firefox/abcd1234.default-release/prefsCleaner.sh: line 1: 404:: command not found

This was quite bizarre, as line 1 of prefsCleaner.sh does not contain "404". It turns out that the problem was that prefsCleaner.sh tries to download and replace its current version with its latest version from GitHub and then runs the replacement, and I had the bad luck of running the script during a GitHub outage, so the script was replacing itself with a file consisting solely of the line 404: Not Found, and then trying to run that :|

GitHub outages are obviously not Arkenfox's fault, but Arkenfox should really handle them more gracefully and robustly than this, i.e., the script should check for HTTP failure before proceeding to replace itself with the HTTP response and trying to run the new version.