Error response format differs from the convention used by other ORS backends

The format of errors returned by the pois API differs from the one used by the rest of ORS endpoints. It is an object with `code` and `message` at top level

```
{
"code": 4000,
"message": "Invalid JSON object in request"
}
```

rather than a JSON object with `code` and `message` nested in `error` such as for directions, for example:
```
{
"error": {
"code": 2001,
"message": "Parameter 'profile' is missing."
},
"info": {
"engine": {
"version": "4.5.0",
"build_date": "2018-03-26T13:41:46Z"
},
"timestamp": 1528809944677
}
}
```