Add a message to explain conditions on the result of an aggregate are not supported

[PierreSenellart]

Currently, non-terminal aggregates, which are not supported, result in undefined behavior. For example, in the tutorial, the following two queries return undefined content (neither the expected one nor an error message):

• SELECT time, person, Count(room) FROM s GROUP BY time, person having Count(room) > 1;
• SELECT * FROM (SELECT time, person, Count(room) as C FROM s GROUP BY time, person) as T WHERE C > 1;

Until such queries are supported, ProvSQL should display an error message when they are run.

[PierreSenellart]

An error message for the first form has been added in 2781157; the second one is a bit more challenging to detect.