Return Option from Contact::add_or_lookup()

This allows to distinguish exceptions,
such as database errors, from invalid user input.
For example, if the From: field of the message
does not look like an email address, the mail
should be ignored. But if there is a database
failure while writing a new contact for the address,
this error should be bubbled up.

src/mimefactory.rs

@@ -1821,6 +1821,7 @@ mod tests {
             Contact::add_or_lookup(&t, "Dave", "dave@example.com", Origin::ManuallyCreated)
                 .await
                 .unwrap()
+                .unwrap()
                 .0;
 
         let chat_id = ChatId::create_for_contact(&t, contact_id).await.unwrap();