Fix exception in virus scan.

Fix exception in virus scan.

* IMPORTANT: We catch exception derived from Exception, but antivirusScan was
  throwing a string exception. That caused global try/catch to miss that, and
  let sendmail believe than the mail was correctly delivered.

* confparser.readConfiguration now return None if rule file doesn't exist.

* Removed log for absence of rule (either because the user file doesn't exist,
  or because no rule are defined inside it.)
git-archimport-id: frederic@jolliton.com--2004-private/mail-filter--main--0.1.2--patch-3