Convert all errors to ereport

[theory]

Replace all instances of elog() with ereport() with an appropriate error code and always prefixed with pg_clickhouse: . For generic FDW errors, for example:

ereport(ERROR,
		errcode(ERRCODE_FDW_ERROR),
		errmsg("pg_clickhouse: failed to push down %s", text_to_cstring(name))
	);

Consult include/server/utils/errcodes.h for the relevant error codes.