Re: Fix error message for MERGE foreign tables

Hi,

MERGE command does not accept foreign tables as targets.
When a foreign table is specified as a target, it shows error messages
like this:

-- ERROR:  cannot execute MERGE on relation "child1"
-- DETAIL:  This operation is not supported for foreign tables.

However, when a partitioned table includes foreign tables as partitions
and MERGE is executed on the partitioned table, following error message
shows.

-- ERROR:  unexpected operation: 5

The latter error message is unclear, and should be the same as the
former one.
The attached patch adds the code to display error the former error
messages in the latter case.
Any thoughts?