Handle properly error pages returned when fetching assets/checksums.txt

[gorhill]

Github had some problems returning files from https://raw2.github.com/gorhill/httpswitchboard/master/ (it happens sometimes), and the code doesn't expect that anything else can be returned when downloading the checksums.txt file. Need to foolproof the code to ensure that the content of the returned checksums.txt file is sane.

The symptom is that I had a list of nonsensical entries in the About tab.