Improve warning / error message and decorator of Download folder in the download runtimes wizard

The download folder defaults to /tmp folder. That is on Kubuntu and maybe Ubuntu as well a bad choice as only root can write to that folder. The folder /var/tmp/ is the better choice on this OS.

I would propose to use java.io.tmpdir for the default value if possible and check if that is writable for the user. If thats not the case we should have some fallback folder.