(ITS#7492) add "onerr" to back-ldap

[masarati@aero.polimi.it]

Full_Name: Pierangelo Masarati
Version: HEAD
OS: irrelevant
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (2.40.21.25)
Submitted by: ando


back-meta supports "onerr" to determine search behavior when an error is
returned by a remote server; a similar behavior may be desirable for back-ldap.

Right now, when an error occurs, back-ldap stops sending entries and returns
success.  The "onerr stop" directive makes back-ldap return the error instead;
"onerr continue" produces the current behavior.