Inform users that key has expired
Issue #428 (closed) was wrongly created by me due to missing feedback from Schleuder.
It would be great if Schleuder had told me, that the key I'm trying to use with
X-RESEND-ENCRYPTED-ONLY had actually expired. Instead, it just told me
Resending to <firstname.lastname@example.org> failed (0 keys found and unencrypted sending disallowed)