Small doc update

use blacklist.db instead of whitelist.db in blacklist example.
Add on_error=continue feature to Plugins

Added attribute on_error to Plugin base class. If set to continue and the
plugin dies mtpolicyd will continue with processing instead of returning
an error.

  <Plugin fault>
    module = "Faulty"
    on_error = "continue"
  </Plugin>

If this plugin dies mtpolicyd will just log the plugins failure and
continure.