fix: Do not throw when listing vector stores (#2460)

When trying to `list` vector_stores , if we cannot retrieve one, log an
error and return all the ones that are valid.

### Test Plan 
```
pytest -sv --stack-config=http://localhost:8321 tests/integration/vector_io/test_openai_vector_stores.py  --embedding-model all-MiniLM-L6-v2
```
Also tested for `--stack-config fireworks`

llama_stack/distribution/routers/vector_io.py

@@ -163,10 +163,14 @@ class VectorIORouter(VectorIO):
         vector_dbs = await self.routing_table.get_all_with_type("vector_db")
         all_stores = []
         for vector_db in vector_dbs:
+            try:
                 vector_store = await self.routing_table.get_provider_impl(
                     vector_db.identifier
                 ).openai_retrieve_vector_store(vector_db.identifier)
                 all_stores.append(vector_store)
+            except Exception as e:
+                logger.error(f"Error retrieving vector store {vector_db.identifier}: {e}")
+                continue
 
         # Sort by created_at
         reverse_order = order == "desc"