## Problem
When converting `proto::GetPageRequest` into `page_api::GetPageRequest`
and validating the request, errors are returned as `tonic::Status`. This
will tear down the GetPage stream, which is disruptive and unnecessary.
## Summary of changes
Emit invalid request errors as `GetPageResponse` with an appropriate
`status_code` instead.
Also move the conversion from `tonic::Status` to `GetPageResponse` out
into the stream handler.