Fix nil pointer when navigating to bad invite URL

Previously when looking up an invite ID that doesn't exist, the database
call wouldn't communicate its non-existence in a standard way --
returning a nil object and nil error. Now the database call returns a
404 error, so handlers can show the correct page.

database.go

@@ -2234,7 +2234,7 @@ func (db *datastore) GetUserInvite(id string) (*Invite, error) {
 	err := db.QueryRow("SELECT id, max_uses, created, expires, inactive FROM userinvites WHERE id = ?", id).Scan(&i.ID, &i.MaxUses, &i.Created, &i.Expires, &i.Inactive)
 	switch {
 	case err == sql.ErrNoRows:
-		return nil, nil
+		return nil, impart.HTTPError{http.StatusNotFound, "Invite doesn't exist."}
 	case err != nil:
 		log.Error("Failed selecting invite: %v", err)
 		return nil, err