Alerting: Improve validation of query and expressions on rule submit (#53258)

* Improve error messages of server-side expression 
* move validation of alert queries and a condition to eval package

pkg/expr/nodes.go

@@ -99,7 +99,7 @@ func (gn *CMDNode) Execute(ctx context.Context, vars mathexp.Vars, s *Service) (
 func buildCMDNode(dp *simple.DirectedGraph, rn *rawNode) (*CMDNode, error) {
 	commandType, err := rn.GetCommandType()
 	if err != nil {
-		return nil, fmt.Errorf("invalid expression command type in '%v'", rn.RefID)
+		return nil, fmt.Errorf("invalid command type in expression '%v': %w", rn.RefID, err)
 	}
 
 	node := &CMDNode{
@@ -120,10 +120,10 @@ func buildCMDNode(dp *simple.DirectedGraph, rn *rawNode) (*CMDNode, error) {
 	case TypeClassicConditions:
 		node.Command, err = classic.UnmarshalConditionsCmd(rn.Query, rn.RefID)
 	default:
-		return nil, fmt.Errorf("expression command type '%v' in '%v' not implemented", commandType, rn.RefID)
+		return nil, fmt.Errorf("expression command type '%v' in expression '%v' not implemented", commandType, rn.RefID)
 	}
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("failed to parse expression '%v': %w", rn.RefID, err)
 	}
 
 	return node, nil