panic handler

[jordens]

I think we need a custom panic handler, at least for the beginning.
We'd usually want to follow the channel power down sequence but that's risky in a panic handler.
Assuming we currently don't touch the bias (dac7571) other than setting it to max pinchoff let's do the following:

for all channels:

• signal_on_pin.set_low()
• enable_power_pin.set_low()

Then turn on some led(s), print the panic, halt.