blob_sidecars with empty response
**Describe the bug**
eth/v1/beacon/blob_sidecars request of an unexisting blob will answer with an empty array.

**To Reproduce**
Steps to reproduce the behavior:
```shell
curl -X 'GET' \
 'https://BEACON_ENDPOINT/eth/v1/beacon/blob_sidecars/5644698?indices=11' \
 -H 'accept: application/json'
``` 

Response:
```shell
{"data":[]}
```

**Additional context**
Please add the error message instead of an empty array.