- Remove unused THD argument from Diagnostics_area::set_error_status()
    and Diagnostics_area::set_ok_status()

  - Introduce Diagnostics_area::set_error_status(sql_errno) to set
    fatal error state (like OOM).

    Diagnostics_area::set_error_status() with 4 arguments should be called
    from THD only.

  - More comments.

  - Various cleanups and polishing.