Better error reporting for unsupported case like max(x)-min(y)

[galkk]

Issue #, if available: #194, #234

Description of changes:
Better error reporting for such queries

POST /_opendistro/_sql/_explain
{
  "query":"""SELECT DestCountry, dayOfWeek, max(FlightDelayMin) - min(FlightDelayMin) + avg(FlightDelayMin)
    FROM kibana_sample_data_flights
    GROUP BY DestCountry, dayOfWeek
  """

returns now

{
  "error": {
    "reason": "There was internal problem at backend",
    "details": "The complex aggregate expressions are not implemented yet: MAX(FlightDelayMin) - MIN(FlightDelayMin)",
    "type": "SqlFeatureNotImplementedException"
  },
  "status": 503
}

instead of non-sensical:

{
  "error": {
    "reason": "Invalid SQL query",
    "details": "",
    "type": "NullPointerException"
  },
  "status": 400
}

and another example:

                        "SELECT DestCountry, dayOfWeek, log(max(FlightDelayMin))" +
                        "   FROM kibana_sample_data_flights\n" +
                        "   GROUP BY DestCountry, dayOfWeek\n"));
```

witll throw

                  "Function calls of form 'log(MAX(...))' are not implemented yet");


By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[chloe-zh]

I think the code changes for now covers more than cases like max(x) - min(y). It seems the object is always the left one of a binary operation, so it can cover the cases as long as it looks like max(x) + y. Could you implement the cases of x + min(y)-like in this PR as well?

[galkk]

@chloe-zh

The case that you've mentioned already works

POST /_opendistro/_sql/_explain
{
  "query":"""SELECT 1+max(FlightDelayMin)
    FROM kibana_sample_data_flights
    GROUP BY DestCountry, dayOfWeek
  """
}


{
  "error": {
    "reason": "There was internal problem at backend",
    "details": "The complex aggregate expressions are not implemented yet: 1 + MAX(FlightDelayMin)",
    "type": "SqlFeatureNotImplementedException"
  },
  "status": 503
}

[chloe-zh]

LGTM, thanks!