Convert HostPortOrFile error to var (#5058)

Convert "no nameservers found" error on parse.HostPortOrFile() to an
exported var for use with `errors.Is()`.
Signed-off-by: SuperQ <superq@gmail.com>

plugin/pkg/parse/host.go

 package parse
 
 import (
+	"errors"
 	"fmt"
 	"net"
 	"os"
@@ -11,6 +12,9 @@ import (
 	"github.com/miekg/dns"
 )
 
+// ErrNoNameservers is returned by HostPortOrFile if no servers can be parsed.
+var ErrNoNameservers = errors.New("no nameservers found")
+
 // Strips the zone, but preserves any port that comes after the zone
 func stripZone(host string) string {
 	if strings.Contains(host, "%") {
@@ -70,7 +74,7 @@ func HostPortOrFile(s ...string) ([]string, error) {
 		servers = append(servers, h)
 	}
 	if len(servers) == 0 {
-		return servers, fmt.Errorf("no nameservers found")
+		return servers, ErrNoNameservers
 	}
 	return servers, nil
 }