Raise NotYetImplemented when encountering UNION

Summary:
We need to explicitly say that UNION clause isn't supported, otherwise
it gets silently ignored. So for example, `CREATE () UNION CREATE ()`
would create 2 nodes without a hitch. On the other hand,
`RETURN 1 UNION RETURN 2` would complain that there is more than 1
RETURN in the query, which was misleading.

Reviewers: florijan, mislav.bradac

Reviewed By: mislav.bradac

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D976

src/query/frontend/ast/cypher_main_visitor.hpp

@@ -133,6 +133,11 @@ class CypherMainVisitor : public antlropencypher::CypherBaseVisitor {
     return expression;
   }
 
+  antlrcpp::Any visitCypherUnion(
+      CypherParser::CypherUnionContext *ctx) override {
+    throw utils::NotYetImplemented("UNION");
+  }
+
   /**
    * @return Query*
    */