CDRIVER-3969 error when creating uncompleted cursor with no server ID (#1321)

* fix type of BCON_INT32 value

* require serverID for open cursor with `mongoc_cursor_new_from_command_reply_with_opts`

* document expectations of server ID option

* assert `found` after `mongoc_cursor_error`

If an error occurs, `found` is false. Assert on the error first to print the error.

* remove unnecessary nested `if`