Add ability to abort scans on RX errors

The readout is now stopped when an RX error (thus far only from discard errors) occur. To stop a mask loop one has to add these lines into the loop:

if self.fifo_readout.stop_readout.is_set():
    break

To prevent an abort on long running scans one can specify:

abort_on_rx_error: False

in the test bench yaml.