rescue the http fetch so timeouts do not abort everything prematurely

I was trying to figure out why I was missing some files in my rubygems mirror. It is because about 1 in 10000 http requests timeout and the whole mirror process aborts when that occurs (it is an exception thrown by Net::HTTP).

This is a simple generic rescue for the HTTP request.

After I made this change, I got a complete mirror for the first time.