Fixing save dialog to be more helpful and not let user save "lost" files

I put this commit in a different branch / pull request because I'm not sure how Saiku's behavior differs between the CE version of Pentaho and other deployments.  Specifically, the index.html change - in CE, without this, it's impossible to load a Saiku file.  But, I feel like you would have fixed this if it was truly broken in all deployments, so it might be a CE only thing.