SQL: return more understandable error when not grouping by a histogram

[eskibars]

Testing on 6.6.0.

Consider using SQL to query for a 1-minute histogram, e.g. across a default metricbeat index (and the default time field, @timestamp).

GET /_xpack/sql?format=txt
{
  "query": """
  SELECT HISTOGRAM("@timestamp", INTERVAL 1 MINUTE) AS h FROM "metricbeat-*" GROUP BY h
  """
}

This correctly returns. However, when omitting GROUP BY ... as in:

GET /_xpack/sql?format=txt
{
  "query": """
  SELECT HISTOGRAM("@timestamp", INTERVAL 1 MINUTE) FROM "metricbeat-*"
  """
}

We get:

{
  "error": {
    "root_cause": [
      {
        "type": "sql_illegal_argument_exception",
        "reason": "Unknown output attribute HISTOGRAM(@timestamp){g->2644358}#2644358"
      }
    ],
    "type": "sql_illegal_argument_exception",
    "reason": "Unknown output attribute HISTOGRAM(@timestamp){g->2644358}#2644358"
  },
  "status": 500
}

We should clean up the error here so it's easier for a user to understand what they've done wrong.

[elasticmachine]

Pinging @elastic/es-search

[astefan]

master(7.0.0): 6968f09
6.x (6.7.0): 4cf0391
6.6 (6.6.1): d05c24c