Report slow control transaction errors from VFAT ADC reads

At the moment, the slow control transaction errors, including timeouts, are not clearly reported to the user. This leads, for example, to "flat" DAC scans, the routine reading constantly the same, stall, value.

Multiple solutions can be envisioned to clearly report the error message. From the most to the least preferred:

1. Use a Zynq firmware with a reasonably increased AXI timeout and drop the ADC cache feature
2. Store the VFAT slow-control transaction error in the BRAM and report it during access to cache registers.
3. Lock the VFAT ADC transactions and monitor the VFAT error counters.
4. Detect the error condition by comparing the cached register value against 0.