Revert "sensor: remove exception in error path"

This reverts commit 379b11316d55b114bdb1bfc5a49839b09ade9038, bringing
it back into line with expected ASIO callback behavior, as documented in
Ic3a36027fa95a77469439b296a1497634cfe030f. The reverted commit was based
on a misunderstanding of the ASIO error handling path.

Tested: With accompanying sdbusplus change, was able to override sensor
value only when ValidationUnsecure mode was enabled, and proper error
response was returned:

root@openbmc:~# busctl set-property xyz.openbmc_project.IpmbSensor \
  /xyz/openbmc_project/sensors/temperature/SSB_Temp \
  xyz.openbmc_project.Sensor.Value Value d 52
Failed to set property Value on interface xyz.openbmc_project.Sensor.Value: Not allowed to set property value.
root@openbmc:~# busctl set-property xyz.openbmc_project.SpecialMode \
  /xyz/openbmc_project/security/special_mode \
  xyz.openbmc_project.Security.SpecialMode SpecialMode s \
  xyz.openbmc_project.Control.Security.SpecialMode.Modes.ValidationUnsecure
root@openbmc:~# busctl set-property xyz.openbmc_project.IpmbSensor \
  /xyz/openbmc_project/sensors/temperature/SSB_Temp \
  xyz.openbmc_project.Sensor.Value Value d 52
root@openbmc:~#

Change-Id: I05c41318954d5d1549752c5ef2c227f3f22a45d3
Signed-off-by: Jonathan Doman <jonathan.doman@intel.com>