s4-ldb: added ldb_error() and ldb_operr()

s4-ldb: added ldb_error() and ldb_operr()

These will be used to help avoid the problem we have with hundreds of
places that do "return LDB_ERR_OPERATIONS_ERROR" without an
explanation. It is very difficult to track down ldb errors which don't
have any explanation.

By replacing "return LDB_ERR_OPERATIONS_ERROR;" with "return ldb_operr(ldb);"
we at least get a file:line message in the ldb error string. It isn't
an ideal error message, but it is much better than just "operations
error"

This change also makes ldb_oom() return the error code
(LDB_ERR_OPERATIONS_ERROR) so you can do:

  return ldb_oom(ldb);

instead of:

  ldb_oom(ldb);
  return LDB_ERR_OPERATIONS_ERROR;