Update the ClamAV default behaviour when an error occurs

MDL-67545 introduced a nice option for what to do if trying to scan a file with Clam AV fails.

This is so much better than all the other options that I think it should be default.

The three options are:

• Treat files as OK - this is risky. You might end up distributing infected files.
• Treat files like viruses - this scares users. It tells them they have a virus when in fact it was just our server having a hiccup.
• Refuse upload, try again - this should surely be the default for new sites?

Well, it seems obvious to me, so I am going to make a patch.