postgres_fdw: Report warning when timeout expires while getting query result.

postgres_fdw: Report warning when timeout expires while getting query result.

When aborting remote transaction or sending cancel request to a remote server,
postgres_fdw calls pgfdw_get_cleanup_result() to wait for the result of
transaction abort query or cancel request to arrive. It fails to get the result
if the timeout expires or a connection trouble happens.

Previously postgres_fdw reported no warning message even when the timeout
expired or a connection trouble happened in pgfdw_get_cleanup_result().
This could make the troubleshooting harder when such an event occurred.

This commit makes pgfdw_get_cleanup_result() tell its caller what trouble
(timeout or connection error) occurred, on failure, and also makes its caller
report the proper warning message based on that information.

Author: Fujii Masao
Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/15aa988c-722e-ad3e-c936-4420c5b2bfea@oss.nttdata.com