Sorry...was not near a PC earlier and didn't remember the specific details.
The master's status structure is $EthIOMaster. There is a mask with flags of all 16 slave warnings ($EthIOMaster.SlaveWarnings), all 16 slave errors ($EthIOMaster.SlaveErrors), and individual error codes for each slave in the form of $EthIOMaster.SlaveXError.
You can also view these same codes from the Ethernet I/O Monitor which is accessible from either the Dashboard (under I/O, left click Ethernet I/O Master and select the monitor) or from the Debug menu, under Ethernet I/O Monitor.