New config option for ClamAV scanning errors to prompt user to try again later

To check this improvement it is necessary to make ClamAV available but return a failure (not a found virus) on scanning an uploaded file. This is easily achievable, if a bit fiddly: As admin on a test site where you have full control, check the admin settings for ClamAV (Site admin > Plugins > Antivirus plugins > Manage) so ClamAV is available. Click on settings for ClamAV. Enable Command line Running method and add a suitable path in the command line setting (/usr/bin/clamscan). Also set the On ClamAV failure setting to Refuse upload, try again. Check that it is possible to upload an image somewhere on your site. (It may be slow uploading the image because the command line virus scan takes time to 'warm up'.) Now go back to the ClamAV admin settings page, and add a text character to the command line scan path (something like /usr/bin/clamscanxxx). This just makes the virus scan manager throw a SCAN_RESULT_ERROR, because the path is incorrect - just what we need for testing the message to the user. Check that when trying to upload an image now you get an onscreen message saying 'There is a problem with AntiVirus scanning at the moment. Your file {$filename} has not been uploaded. Please try again later.' Check you also get an admin email with details of the ClamAV failure. Finally reverse the changes to your admin antivirus settings.  There is a phpunit test as well.