PS: segreq handler should log more about fetcher errors and reply with NACK/error

[lukedirtwalker]

If the segfetcher logic in the PS returns an error, the handler silently returns, we should log the error and return a negative reply to the client.

Problematic code:

scion/go/path_srv/internal/segreq/handler.go

Lines 70 to 74 in cd316a3

	segs, err := h.fetcher.FetchSegs(ctx,
	segfetcher.Request{Src: segReq.SrcIA(), Dst: segReq.DstIA()})
	if err != nil {
	return infra.MetricsErrInternal
	}

and missing logging in:

scion/go/path_srv/internal/segreq/handler.go

Lines 87 to 89 in cd316a3

	if err = rw.SendSegReply(ctx, reply); err != nil {
	return infra.MetricsErrInternal
	}