FIX-OCT-1572: Ignore errors when 404 (#167)

## Description

## Definition of Done

1. [ ] Acceptance criteria are met.
2. [ ] PR is manually tested before the merge by developer(s).
    - [ ] Happy path is manually checked.
3. [ ] PR is manually tested by QA when their assistance is required
(1).
- [ ] Octant Areas & Test Cases are checked for impact and updated if
required (2).
4. [ ] Unit tests are added unless there is a reason to omit them.
5. [ ] Automated tests are added when required.
6. [ ] The code is merged.
7. [ ] Tech documentation is added / updated, reviewed and approved
(including mandatory approval by a code owner, should such exist for
changed files).
    - [ ] BE: Swagger documentation is updated.
8. [ ] When required by QA:
    - [ ] Deployed to the relevant environment.
    - [ ] Passed system tests.

---

(1) Developer(s) in coordination with QA decide whether it's required.
For small tickets introducing small changes QA assistance is most
probably not required.

(2) [Octant Areas & Test
Cases](https://docs.google.com/spreadsheets/d/1cRe6dxuKJV3a4ZskAwWEPvrFkQm6rEfyUCYwLTYw_Cc).

Author: kgarbacinski

backend/app/modules/multisig_signatures/core.py

@@ -1,8 +1,10 @@
 import json
+from http import HTTPStatus
 from typing import List, Tuple, Dict
 
 from eth_account.messages import SignableMessage
 
+from app.exceptions import ExternalApiException
 from app.infrastructure.database.models import MultisigSignatures
 from app.infrastructure.external_api.safe.message_details import get_message_details
 from app.infrastructure.external_api.safe.user_details import get_user_details
@@ -62,17 +64,20 @@ def approve_pending_signatures(
             approved_signatures.append(_create_signature_object(pending_signature))
             continue
 
-        message_details = get_message_details(  # TODO extract API call to service
-            pending_signature.msg_hash, is_mainnet=is_mainnet
-        )
-        confirmations = message_details["confirmations"]
-        threshold = int(
-            get_user_details(
+        try:
+            message_details = get_message_details(  # TODO extract API call to service
+                pending_signature.msg_hash, is_mainnet=is_mainnet
+            )
+            user_details = get_user_details(
                 pending_signature.address, is_mainnet=is_mainnet
-            )[  # TODO extract API call to service
-                "threshold"
-            ]
-        )
+            )  # TODO extract API call to service
+        except ExternalApiException as e:
+            if e.status_code == HTTPStatus.NOT_FOUND:
+                continue
+            raise e
+
+        confirmations = message_details["confirmations"]
+        threshold = int(user_details["threshold"])
 
         if len(confirmations) >= threshold:
             pending_signature.confirmed_signature = message_details["preparedSignature"]