Handle servers being down more gracefully

[ctrueden]

The sites.imagej.net server went down earlier today, resulting in Fiji bug #807.

As reported by Graeme Ball:

The Fiji updater (Help->Update Fiji) seems to hang, but after a l ... o ... n ... g wait, reports "Could not update from site blah" where blah is 3D ImageJ Suite, BioVoxxel and Cookbook; all of which are at sites.imagej.net according to "Show Details".

Since so many sites are actually the same server, it would be nice to time out only once, and then just skip the others. This would reduce the l ... o ... n ... g wait, and while at it we could also give a nicer error message such as "The server sites.imagej.net seems to be down. Please try running the updater again later."