Refactor and adhere more to the YubiKSM communication spec.
Refactor and adhere more to the YubiKSM communication spec.

Errors should not be returned as HTTP non-2xx, but as a HTTP 200
with a body containing the string

ERR optional-reason

Yubico/python-pyhsm,ThomasHabets/python-pyhsm,ThomasHabets/python-pyhsm,ThomasHabets/python-pyhsm