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

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

**Background issue**: https://github.com/influxdata/docs.influxdata.com/issues/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._

Search the repository for the existing "mixing aggregate and non-aggregate queries is not supported" error text and inspect the surrounding parser or error-handling code. Done means the message clearly identifies the SELECT-clause restriction described in the issue, with any relevant existing tests still passing.

Written by the indexing model from the issue text.