[Query] execute long prepare first show explain error until execution is complete

To repro:

• from query/UDF create function: 

  function sleep(delay) {

  var start = new Date().getTime();

  while (new Date().getTime() < start + delay);

  return delay;

  }

• create a long running prepare statement 

  prepare sleep1min as select sleep(60000);

• from query UI execute statement 

  execute sleep1min

You will first see "explain" error but query is still executing:

Then result will finally show:

This might be confusing for user and think the prepare statement failed to execute.