Sidecar sequencer panics as recovery when front I/O not programmed.

I think we'd like to get some better error messages here than just a panic, especially since debugging this in MFG is basically impossible today.

We get in the station logs:

humility: attached to 0483:3754:0039001D4741500920383733 via ST-Link V3
Sequencer.set_tofino_seq_policy() => Err(<server died; its new ID is 26>)

manually debugging, we set the state disposition to hold:
humility jefe -H sequencer

gets us

humility tasks
D TASK                       GEN PRI STATE
 0 jefe                         0   0 recv, notif: fault timer(T+12)
 1 sys                          0   1 recv, notif: exti-wildcard-irq(irq6/irq7/irq8/irq9/irq10/irq23/irq40)
 2 rng_driver                   0   6 recv
 3 update_server                0   3 recv
 4 auxflash                     0   3 recv
 5 net                          0   5 wait: send to sequencer/gen16
 6 control_plane_agent          0   7 wait: send to net/gen0
 7 sprot                        0   4 recv
 8 udpecho                      0   6 wait: send to net/gen0
 9 udpbroadcast                 0   6 wait: send to net/gen0
10 monorail                     0   6 wait: send to sequencer/gen16
11 i2c_driver                   0   2 recv
12 hiffy                        0   5 notif: bit31(T+209)
13 sensor                       0   4 recv
14 ecp5_mainboard               0   3 recv
15 ecp5_front_io                0   3 recv
16 transceivers                 0   6 wait: send to net/gen0
17 packrat                      0   3 recv
18 sequencer                 1168   4 FAULT: panicked at drv/sidecar-seq-server/src/main.rs:963:35:
explicit panic (was: ready)
19 thermal                      0   5 wait: send to sequencer/gen16
20 power                      390   6 wait: send to sequencer/gen16
21 validate                     0   5 recv
22 ignition                     0   5 wait: send to sequencer/gen16
23 vpd                          0   3 recv
24 dump_agent                   0   6 wait: send to net/gen0
25 idle                         0   8 RUNNING
26 udprpc                       0   6 wait: send to net/gen0

so the panic comes from here: front_io_board.init().unwrap_lite();

and the sequencer ringbuff shows:

humility: attached to 0483:3754:0039001D4741500920383733 via ST-Link V3
humility: ring buffer drv_oxide_vpd::__RINGBUF in sequencer:
humility: ring buffer drv_packrat_vpd_loader::__RINGBUF in sequencer:
humility: ring buffer drv_sidecar_seq_server::__RINGBUF in sequencer:
 NDX LINE      GEN    COUNT PAYLOAD
   0  825        1        1 FpgaInit
   1  868        1        1 SkipLoadingBitstream
   2  878        1        1 MainboardControllerId(0x1de5bae)
   3  892        1        1 MainboardControllerChecksum(0x6407475e)
   4  924        1        1 MainboardControllerVersion(0x283)
   5  925        1        1 MainboardControllerSha(0x3c8d1c33)
   6  926        1        1 FpgaInitComplete
   7   31        1        1 LoadingClockConfiguration
   8  951        1        1 ClockConfigurationComplete
   9  956        1        1 FrontIOBoardPresent
  10   81        1        1 LoadingFrontIOControllerBitstream { fpga_id: 0x0 }
  11   86        1        1 FpgaBitstreamError(0x300)

This ended up being a broken ARF6 cable between main and the front-io but this is an extremely annoying issue to debug at the MFG station so getting some better visibility/ error reporting here will help.

Start in drv/sidecar-seq-server/src/main.rs at the front_io_board.init().unwrap_lite() call around line 963. Read the sequencer ring-buffer events showing FrontIOBoardPresent and FpgaBitstreamError, then trace how this initialization failure is reported. Done means a failed front-I/O initialization provides useful diagnostic error information instead of only panicking.