Move table open result processing to the caller

* st_schema_table::process_table doesn't have to check whether the table
was opened successfully
* It also doesn't have to check for a thd error and convert it to a warning
* This simplifies adding new tables into information_schema
* A callback still can output some info to a user in case of error. In
order to do this, I_S_EXTENDED_ERROR_HANDLING should be specified in
i_s_requested_object.