/v1/catalog/service/:service_name 404

#### Feature Description

Return 404 when `service_name` does not exist in catalogue query `/v1/catalog/service/:service_ name`

As of today, consul returns `200` whether the service exists or not — returning an empty result set, `[]`, if the service isn't present

#### Use Case(s)

I want to handle the presence or absence of a service differently, and using the HTTP status code would be easier than matching on an empty result set

Begin by locating the Go implementation of GET /v1/catalog/service/:service_name and tracing how a missing service is handled. Done means an absent service returns HTTP 404 rather than 200 with an empty result set, while an existing service continues to return its catalog response.

Written by the indexing model from the issue text.