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/test_utils.rs

@@ -529,7 +529,11 @@ impl TestContext {
         let (contact_id, modified) =
             Contact::add_or_lookup(self, &name, &addr, Origin::MailinglistAddress)
                 .await
-                .unwrap();
+                .expect("add_or_lookup")
+                .expect(&format!(
+                    "contact with address {:?} cannot be created",
+                    &addr
+                ));
         match modified {
             Modifier::None => (),
             Modifier::Modified => warn!(&self.ctx, "Contact {} modified by TestContext", &addr),