ipnlocal: log failure to get ssh host keys

When reporting ssh host keys to control, log a warning
if we're unable to get the SSH host keys.

Updates tailscale/escalations#21

Signed-off-by: Percy Wegmann <percy@tailscale.com>

ipn/ipnlocal/ssh_stub.go

@@ -11,8 +11,8 @@ import (
 	"tailscale.com/tailcfg"
 )
 
-func (b *LocalBackend) getSSHHostKeyPublicStrings() []string {
-	return nil
+func (b *LocalBackend) getSSHHostKeyPublicStrings() ([]string, error) {
+	return nil, nil
 }
 
 func (b *LocalBackend) getSSHUsernames(*tailcfg.C2NSSHUsernamesRequest) (*tailcfg.C2NSSHUsernamesResponse, error) {