[settings] Expose exit status of failed command via ${errno}

[settings] Expose exit status of failed command via ${errno}

Allow scripts to report errors in more detail by exposing the most
recent error via the ${errno} setting.  For example:

    chain ${filename} || goto failed
    ...
    :failed
    imgfree http://192.168.0.1/ipxe_error.php?error=${errno}

Note that ${errno} is valid only immediately after executing a failed
command.

Signed-off-by: Michael Brown <mcb30@ipxe.org>