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.

python/tests/test_1_online.py

@@ -939,6 +939,20 @@ def test_dont_show_emails(acfactory, lp):
             ac1.get_config("configured_addr")
         ),
     )
+    ac1.direct_imap.append(
+        "Spam",
+        """
+        From: delta<address: inbox@nhroy.com>
+        Subject: subj
+        To: {}
+        Message-ID: <spam.message99@junk.org>
+        Content-Type: text/plain; charset=utf-8
+
+        Unknown & malformed message in Spam
+    """.format(
+            ac1.get_config("configured_addr")
+        ),
+    )
     ac1.direct_imap.append(
         "Spam",
         """