ambiguous status codes makes it difficult to reason about api errors

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#### Overview of the Issue

When calling the API to POST a prepared query, it returns 500 if it already exists. The correct would be to return 409 Conflict.

The body message makes it clear that the query already has been created but the 500 return code is less than ideal.

Not only it's semantically wrong but it also makes it harder to reason with in automated logic as 5xx is reserved for Server Errors.

#### Reproduction Steps

1. Call

```bash
curl http://127.0.0.1:8500/v1/query \
--request POST \
--data \
'{
"Name": "test",
"Service": {
"Service": "test",
"Failover": {
"NearestN": 2
}
}
}'
```

2. Then call it again.