s4-dsdb: use ldb_operr() in the dsdb code

s4-dsdb: use ldb_operr() in the dsdb code

this replaces "return LDB_ERR_OPERATIONS_ERROR" with "return ldb_operr(ldb)"
in places in the dsdb code where we don't already explicitly set an
error string. This should make is much easier to track down dsdb
module bugs that result in an operations error.