SMART — transmission of in-band error codes

It is unclear how the in-band error codes are transmitted in an MS round. This
should be clarified by adding the following paragraph to Section 2.3.3:
“The check byte is also used for transmitting inline error codes. In case of
an error, all four data bytes of the MSD-Round are set to 0xFF and the
check byte contains an error code in the lower nibble while the bits of the
higher nibble are all set. Note that a message containing the value
0xFFFFFFFF differs significantly from an error message in the check byte.”

Accept changes as proposed.