Warn / raise for returning() / return_defaults() combinations

Warn / raise for returning() / return_defaults() combinations

A warning is emmitted if a returning() method such as
:meth:`_sql.Insert.returning` is called multiple times, as this does not
yet support additive operation.  Version 1.4 will support additive
operation for this.  Additionally, any combination of the
:meth:`_sql.Insert.returning` and :meth:`_sql.Insert.return_defaults`
methods now raises an error as these methods are mutually exclusive;
previously the operation would fail silently.

Fixes: #5691
Change-Id: Id95e0f9da48bba0b59439cb26564f0daa684c8e3