add ExternalServerError in error codes to retry for backfilling
Currently, we have 856 cached records with error ExternalServerError. This error is thrown when connection to Spawning AI API is not working:

`Error when trying to connect to https://opts-api.spawningaiapi.com/api/v2/query/urls: '{'detail': 'An unknown error has occurred.'}'`

We should retry those jobs because it is not related to an error with our logic but with the connection.
```
Atlas atlas-x5jgb3-shard-0 [primary] datasets_server_cache> db.cachedResponsesBlue.countDocuments({error_code:"ExternalServerError"})
856
Atlas atlas-x5jgb3-shard-0 [primary] datasets_server_cache> db.cachedResponsesBlue.distinct("kind", {error_code:"ExternalServerError"})
[ 'split-opt-in-out-urls-count', 'split-opt-in-out-urls-scan' ]
```
-------------------------
Also removing NoIndexableColumnsError from the list since the exception no longer exist on the code and all the old entries have been refreshed.
```
Atlas atlas-x5jgb3-shard-0 [primary] datasets_server_cache> db.cachedResponsesBlue.countDocuments({error_code:"NoIndexableColumnsError"})
0
```
add ExternalServerError in error codes to retry for backfilling: Currently, we have 856 cached records with error ExternalServerError. This error is thrown when connection to Spawning AI API is not working:

`Error when trying to connect to https://opts-api.spawningaiapi.com/api/v2/query/urls: '{'detail': 'An unknown error has occurred.'}'`

We should retry those jobs because it is not related to an error with our logic but with the connection.
```
Atlas atlas-x5jgb3-shard-0 [primary] datasets_server_cache> db.cachedResponsesBlue.countDocuments({error_code:"ExternalServerError"})
856
Atlas atlas-x5jgb3-shard-0 [primary] datasets_server_cache> db.cachedResponsesBlue.distinct("kind", {error_code:"ExternalServerError"})
[ 'split-opt-in-out-urls-count', 'split-opt-in-out-urls-scan' ]
```
-------------------------
Also removing NoIndexableColumnsError from the list since the exception no longer exist on the code and all the old entries have been refreshed.
```
Atlas atlas-x5jgb3-shard-0 [primary] datasets_server_cache> db.cachedResponsesBlue.countDocuments({error_code:"NoIndexableColumnsError"})
0
```