Mavericks failed to download
Quite a lot of traffic going through Apple’s servers with the announcement that everyone with Snow Leopard or later can download Mavericks for free yesterday (the list of compatible machines is the same as for Mountain Lion, see here to check if yours qualifies).
This can be quite annoying as the download can go right up until the final few minutes and then return the error.
Alas, if you keep getting the ‘failed to download’ message, the only advice is to sit and wait it out till traffic subsides a little, I’m afraid. If you haven’t done so already, use the time to make a backup of your system and clear out your login items (System Preferences > Users & Groups (or Accounts for SL users) | Login Items. For more general advice on what to do to ensure a smooth upgrade, read the tips here.
Note: if you’re getting the ‘This copy of Mavericks can’t be verified’ message while using an USB installer, have a look here for a likely explanation and solution.