- Adding "return true" into LEX::set_system_variable()
  and LEX::set_default_system_variable() after my_error().
  This makes the parser exit on error immediately.
  Previously, the error was caught only in mysql_parser(),
  a few lines after the parse_sql() call.
- Fixing "--error 1272" to "--error ER_VARIABLE_IS_NOT_STRUCT" in tests