err: mixing aggregate and non-aggregate queries is not supported

[kelseiv]

Hi, customer requested clarification on the following InfluxQL error:
err: mixing aggregate and non-aggregate queries is not supported.

Background issue: influxdata/docs.influxdata.com-ARCHIVE#2654

Curious, if this error is specific to SELECT, could we update to:
err: SELECT clause cannot include an aggregate function and a non-aggregate function, field key, or tag key.