"humility pmbus" should fail a little more gracefully on non-PMBus device

Due to some bad advice from yours truly, we attempted to treat a MAX31790 as a PMBus device on Gimlet:

$ humility pmbus -c 4 -d 0x20 -D adm1272
humility: attached via ST-Link
humility: pmbus failed: request failed: Some(Fault(ReturnStackOverflow))

Here is the scan of the device:

$ humility i2c -s -c 4 -d 0x20
humility: attached via ST-Link

Register scan for device 0x20 on I2C4:

      - = No register        ! = No device        X = Timed out

ADDR  0x0 0x1 0x2 0x3 0x4 0x5 0x6 0x7 0x8 0x9 0xa 0xb 0xc 0xd 0xe 0xf
0x00   27  bb  20  20  20  20  20  20  4c  4c  4c  4c  4c  4c  00  00
0x10   00  00  3f  3f  45  00  00  00  ff  e0  ff  e0  ff  e0  ff  e0
0x20   ff  e0  ff  e0  ff  e0  ff  e0  ff  e0  ff  e0  ff  e0  ff  e0
0x30   ff  81  ff  81  ff  81  ff  81  ff  81  ff  81  00  00  00  00
0x40   ff  80  ff  80  ff  80  ff  80  ff  80  ff  80  00  00  00  00
0x50   3c  00  3c  00  3c  00  3c  00  3c  00  3c  00  00  00  00  00
0x60   00  00  00  00  00  00  00  00  ff  ff  ff  ff  ff  ff  ff  ff
0x70   ff  ff  ff  ff  ff  ff  ff  ff  00  00  00  00  00  00  00  00
0x80   ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff
0x90   ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff
0xa0   ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff
0xb0   ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff
0xc0   ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff
0xd0   ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff
0xe0   ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff
0xf0   ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff  ff

We are dying on a return stack overflow because there are several ffs in there that are being treated as block lengths -- and it doesn't take too many of those to overflow the return stack. We should fail a little more gracefully in this case, and in particular, we know from earlier reads that this can't possibly be a PMBus device.