merge MMcM: I didn't understand how to check warnings.

Merge pull request #1027 from MMcM/sql-layer/isolation-yaml-warn
https://github.com/FoundationDB/sql-layer/pull/1027
merge MMcM: Improve handling of subqueries in an aggregate query

Merge pull request #1028 from MMcM/sql-layer/sql-subquery-with-group-by-column
https://github.com/FoundationDB/sql-layer/pull/1028
Give an error for old protocol version.
Special handling of error message to V2 client.
Add an IT for that.