in THD.
In future the error may be stored elsewhere (not in net.report_error) and 
it's important to start using an opaque getter to simplify merges.


sql/filesort.cc:
  net.report_error -> is_error()
sql/ha_ndbcluster_binlog.cc:
  net.report_error -> is_error()
sql/item_func.cc:
  net.report_error -> is_error()
sql/item_subselect.cc:
  net.report_error -> is_error()
sql/set_var.cc:
  net.report_error -> is_error()
sql/sp.cc:
  net.report_error -> is_error()
sql/sp_head.cc:
  net.report_error -> is_error()
sql/sql_base.cc:
  net.report_error -> is_error()
sql/sql_class.cc:
  net.report_error -> is_error()
sql/sql_class.h:
  net.report_error -> is_error()
sql/sql_connect.cc:
  net.report_error -> is_error()
sql/sql_delete.cc:
  net.report_error -> is_error()
sql/sql_insert.cc:
  net.report_error -> is_error()
sql/sql_parse.cc:
  net.report_error -> is_error()
sql/sql_prepare.cc:
  net.report_error -> is_error()
sql/sql_select.cc:
  net.report_error -> is_error()
sql/sql_union.cc:
  net.report_error -> is_error()
sql/sql_update.cc:
  net.report_error -> is_error()
sql/sql_view.cc:
  net.report_error -> is_error()
sql/sql_yacc.yy:
  net.report_error -> is_error()