🐛(dimail) grab duplicate displayname error

OpenXchange's primary key is display name (= first name + last name).
In absence of clear error message from dimail (yet), we catch errors 500 and
check if they're not due to the display name already existing in the context

src/backend/mailbox_manager/utils/dimail.py

@@ -165,6 +165,35 @@ class DimailAPIClient:
                 "Permission denied. Please check your MAIL_PROVISIONING_API_CREDENTIALS."
             )
 
+        # Dimail doesn't return a clear error for duplicate yet
+        # but a 500 internal error
+        if response.status_code == status.HTTP_500_INTERNAL_SERVER_ERROR:
+            try:
+                address = session.get(
+                    f"{self.API_URL}/domains/{mailbox.domain.name}/address/{mailbox.local_part}/",
+                    json=payload,
+                    headers=headers,
+                    verify=True,
+                    timeout=self.API_TIMEOUT,
+                )
+            except requests.exceptions.ConnectionError as error:
+                logger.error(
+                    "Connection error while trying to reach %s.",
+                    self.API_URL,
+                    exc_info=error,
+                )
+                raise error
+
+            if address.status_code == status.HTTP_200_OK:
+                primary = address.json()["ox_primary_email"]
+                raise exceptions.ValidationError(
+                    {
+                        "NON_FIELD_ERRORS": [
+                            f"First name + last name combination already in use in this context : {primary}."
+                        ]
+                    }
+                )
+
         return self.raise_exception_for_unexpected_response(response)
 
     def create_user(self, user_id):