If update fails, re-trying should try already-downloaded update first

[andymatuschak]

**Chris Hanson (eschaton)* reported (on Launchpad) on' 2009-02-05:*

When an update is downloaded but installing the update fails (say with a
crash), the next time you try to update Sparkle should first attempt to
use the already-downloaded update if it still exists, is still the
latest, and its signature is intact.

[andymatuschak]

**Andy Matuschak (andymatuschak)* wrote on 2009-02-04:*

Very true. I won't get to it anytime soon, though, I'm afraid.

[andymatuschak]

**Stuart Morgan (stuart-morgan)* wrote on 2009-04-26:*

This would also help the case where someone downloads the update but
typos their password while trying to authorize (there's apparently only
one try, then you have to re-download), which a Camino user on a
satellite connection has complained about.

[kornelski]

We can do that as part of #607