Standardize 4xx error responses

As the maintainer of the dandi-api client, I would greatly appreciate it if all 4xx responses had bodies of the form:

```json
{
"error": "PermissionDeniedError",
"message": "You do not have permission to delete this asset."
}
```

where the `error` values belong to a fixed, documented set so that the client can map them to custom Python exception types.