Handle missing package gently

Right now if a package is missing from the archive when downloading, you hit a 404 that creates an exception:

System.Net.WebException: The remote server returned an error: (404) Not Found.

Ideally, we should catch this exception and present this error nicely instead of crashing.

Basically something better than this, but, on DownloadPretty.cs:

            try
            {
                await downloadTask;
            }
            catch (Exception ex)
            { 
              
                return false;
            }

            return true;