Bad `pmbus -r` output for missing sensor

If a sensor is missing from the bus, pmbus -r just exits silently, instead of printing something about missing data.

Here's the full sensor list:

λ humility pmbus -s
humility: attached via ST-Link V3
DEVICE        RAIL             PG? #FLT       VIN      VOUT      IOUT    TEMP_1
tps546b24a    V3P3_SP_A2         Y    2    11.98V     3.31V     0.29A   30.25°C
tps546b24a    V3P3_SYS_A0        Y    0    11.98V     3.31V    -0.48A   31.50°C
tps546b24a    V5_SYS_A2          Y    2    12.00V     4.99V     0.31A   31.75°C
tps546b24a    V1P8_SYS_A2        Y    0    11.98V     1.79V     0.53A   29.50°C
raa229618     VDD_VCORE          Y    0    12.05V     0.99V    20.20A   42.00°C
raa229618     VDD_MEM_ABCD       Y    0    12.07V     1.22V    25.30A   43.00°C
raa229618     VDDCR_SOC          Y    0    11.98V     0.96V    20.60A   39.00°C
raa229618     VDD_MEM_EFGH       Y    0    11.98V     1.23V    26.30A   40.00°C
isl68224      VPP_ABCD           Y    0    11.96V     2.50V     0.10A   34.00°C
isl68224      VPP_EFGH           Y    0    11.96V     2.50V     0.20A   36.00°C
isl68224      V1P8_SP3           Y    0    11.99V     1.80V     1.10A    0.00°C
adm1272       V54_FAN            -    -         -         -         -         -
adm1272       V54_HS_OUTPUT      Y    1    0x088d    0x088c    0x08b6   32.36°C
tps546b24a    V0P96_NIC_VDD_A0HP   N    0    11.98V     0.01V    -0.13A   32.25°C
bmr491        V12_SYS_A2         Y    1    53.62V    12.02V    11.25A   52.00°C

If you ask for details on V54_FAN, here's what happens:

λ humility pmbus -r V54_FAN
humility: attached via ST-Link V3

(...that's it)

Reported by @Aaron-Hartwig

Start by locating the implementation and tests for the pmbus -r command, then reproduce the V54_FAN case from the issue. Trace how a missing sensor is handled after the command attaches. Done means requesting a missing sensor reports missing data instead of exiting silently, with coverage for that behavior.