Add a Retry button when some packages fail to download

Almost every time that "some packages fail to download" trying again succeeds in retreiving those packages. It's very rare that the file is actually missing or the server is down compared to the number of times that it glitches. Though I do not know if that is my networks fault or not... =)

However, instead of having to hit Cancel, then wait for reload of list, then hit Apply and confirm again (many steps), the warning dialog should provide a "Retry" button that does just that - skips all those steps and just tries again.