add logging to disclaimer action

add logging to disclaimer action

the disclaimer action currently does not log, if a disclaimer got added or not.
given that there are a few not directly obvious cases where a disclaimer does
not get added (e.g. it depends on the mail's encoding) - logging success or
failure should help in debugging

Tested by sending mails, where adding the disclaimer works, and where it fails.
verified that the log-tracker also adds those lines to its output.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>