net/dns: error out on linux if /etc/resolv.conf can't be read.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/manager_linux.go

@@ -23,6 +23,9 @@ func NewOSConfigurator(logf logger.Logf, interfaceName string) (OSConfigurator,
 	if os.IsNotExist(err) {
 		return newDirectManager()
 	}
+	if err != nil {
+		return nil, fmt.Errorf("reading /etc/resolv.conf: %w", err)
+	}
 
 	switch resolvOwner(bs) {
 	case "systemd-resolved":