Unimplemented grpc response missing description

*Title*: *Unimplemented grpc response missing description*

*Description*:
>When Envoy receives a grpc requests for which is has no route, it responds with Unimplemented (12) and an empty description. It would be very helpful to debugging if it provided a description along the lines of "No route available".

*Repro steps*:
> I reproduced this by running the official docker image for both 1.21-latest and 1.22-latest using the config below. I then used grpcui to send a request to the docker image for an rpc it didn't have a route for.

*Config*:
```
admin:
address:
socket_address: { address: 0.0.0.0, port_value: 9901 }

static_resources:
listeners:
- name: listener
address:
socket_address: { address: 0.0.0.0, port_value: 10000 }
filter_chains:
- filters:
- name: envoy.filters.network.http_connection_manager
typed_config:
"@type": type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
stat_prefix: http
codec_type: AUTO
route_config: {}
http_filters:
- name: envoy.filters.http.router
typed_config:
"@type": type.googleapis.com/envoy.extensions.filters.http.router.v3.Router
```