Error condition handling
There are some obvious fault conditions which need to be handled, e.g.
- All WM busy
- WM FIFO full
- WM FIFO not empty at end of WAIT
An interim plan would be:
- Provide a top-level error vector with one bit per condition which could be connected to an AXI monitoring block
- Latch each condition and put a set of bits in the trailer
- Ensure that an empty event with the appropriate trailer bit is set is sent for the "All WM busy" condition.