extensionDownloader: Properly error out when downloading/parsing infos

When the extension downloader was originally designed, the information
downloading part was inserted at the last minute, along with the modal
dialog as a security feature to make sure an extension didn't silently
get installed on the user's machines either due to a security issue in
the browser-plugin, or an XSS issue on the extensions website. Correct
the mistake I made when writing the code; instead of dropping an error
on the floor, log it correctly. This "bug" has already bitten a number
of users who forgot to configure proxy settings in the control center.

https://bugzilla.gnome.org/show_bug.cgi?id=679099