Returns an errornumber for the server (last request) identified by id. The error code is either a Z39.50 diagnostic code (usually a Bib-1 diagnostic) or a client side error code which is generated by PHP/YAZ itself, such as "Connect failed", "Init Rejected", etc.
yaz_errno() should be called after network activity for each server - (after yaz_wait() returns) to determine the success or failure of the last operation (e.g. search). To get a text description of the error, call yaz_error().