net: fix lookupHost to return DNSError on Plan 9

CL 168597 added IsNotFound field to DNSError.
However, this change broke TestLookupNonLDH on Plan 9
because LookupHost is expected to return a DNSError,
while on Plan 9, it returned an error string.

This change fixes the implementation of lookupHost on
Plan 9 to return a DNSError instead of an error string.

Fixes #31672.

Change-Id: Ia805c8965af63ddee7ccfdebb9462a5502b0269d
Reviewed-on: https://go-review.googlesource.com/c/go/+/173857


Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>