email - check leap generated headers for signatures

If an email is encrypted but not signed I think the header added is self.LEAP_SIGNATURE_INVALID. Shouldn't it be self.LEAP_SIGNATURE_COULD_NOT_VERIFY?

If an email is not encrypted is the signature checked?

See _maybe_decrypt_msg at leap/mail/imap/fetch.py

(from redmine: created on 2014-08-18, relates #6914 (closed), relates #7429 (closed))