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>

src/net/lookup_plan9.go

@@ -147,10 +147,12 @@ func (*Resolver) lookupHost(ctx context.Context, host string) (addrs []string, e
 	// host names in local network (e.g. from /lib/ndb/local)
 	lines, err := queryCS(ctx, "net", host, "1")
 	if err != nil {
+		dnsError := &DNSError{Err: err.Error(), Name: host}
 		if stringsHasSuffix(err.Error(), "dns failure") {
-			err = errNoSuchHost
+			dnsError.Err = errNoSuchHost.Error()
+			dnsError.IsNotFound = true
 		}
-		return
+		return nil, dnsError
 	}
 loop:
 	for _, line := range lines {