improve error messages

servers/slapd/back-ldap/add.c

servers/slapd/back-ldap/bind.c

servers/slapd/back-ldap/compare.c

servers/slapd/back-ldap/delete.c

servers/slapd/back-ldap/modify.c

servers/slapd/back-ldap/modrdn.c

servers/slapd/back-ldap/search.c

servers/slapd/back-meta/add.c

servers/slapd/back-meta/compare.c

servers/slapd/back-meta/conn.c

servers/slapd/back-meta/delete.c

servers/slapd/back-meta/modify.c

servers/slapd/back-meta/modrdn.c

servers/slapd/back-meta/search.c