lntest: properly handle shutdown error

This commit removes the panic used in checking the shutdown log.
Instead, the error is returned and asserted in `shutdownAllNodes` so
it's easier to check which node failed in which test. We also catch all
the errors returned from `StopDaemon` call to properly access the
shutdown behavior.

lntest/harness_node_manager.go

@@ -112,11 +112,14 @@ func (nm *nodeManager) registerNode(node *node.HarnessNode) {
 // ShutdownNode stops an active lnd process and returns when the process has
 // exited and any temporary directories have been cleaned up.
 func (nm *nodeManager) shutdownNode(node *node.HarnessNode) error {
+	// Remove the node from the active nodes map even if the shutdown
+	// fails as the shutdown cannot be retried in that case.
+	delete(nm.activeNodes, node.Cfg.NodeID)
+
 	if err := node.Shutdown(); err != nil {
 		return err
 	}
 
-	delete(nm.activeNodes, node.Cfg.NodeID)
 	return nil
 }