Improve response for site not found case

**Current state:**
When a site is not found, users receive a 404 and an HTML response regardless of format requested.

**Test case:**
https://geocoder.api.gov.bc.ca/sites/nearest.json?point=-122.861715%2C49.825979&maxDistance=10

**Proposed future state:**
When a site not found response is given for a ‘near’ or ‘nearest’ request, a ‘No site found’ response is given in the same format as the request (json, geojson etc) with HTTP 200 status code. In some cases, such as SHP format a different approach may be required.

Start by reproducing the nearest.json request at the URL in the issue and trace how a missing site is handled for near or nearest requests. Inspect the response-format paths for JSON, GeoJSON, and other formats, including the noted SHP exception. Done means a missing site returns a format-matched “No site found” response with HTTP 200 where applicable.

Written by the indexing model from the issue text.