Understand/handle SAM FIFO timeouts

Just had this happen, after hundreds of ramps, thousands of reads. Restarting the actor (and thus resetting the ASIC, but not power-cycling anything) fixed the problem. But needs to be figured out. At the very least the timeouts are too long, and a few timeouts in a row should trigger some recovery.

2019-03-15 21:38:21.075Z sam 40 qusb.py:555 FIFO timeout! need=0x00100000, npix=0x00000006, empty=0x0e, full=0x00