There seems no good reason to have both mongoc_server_description_t.error and mongoc_topology_scanner_node_t.last_error.

Note that mongoc_server_description_handle_ismaster takes a bson_error_t* from the scanner node, but doesn't use it. Was this intended as an IN parameter to copy to the server description, or an OUT parameter in case the ismaster response doesn't parse?

Also consider the error "`minheartbeatfrequencyms` not reached yet": perhaps if mongoc_topology_select does no scan it should return the same error it did previously, instead of the generic "`minheartbeatfrequencyms` not reached yet" error.