In previous implementation `SignService` converted all `error` values to
`INTERNAL` server failure status. That was done for simplification only.
There is a need to transmit status errors as corresponding status
messages.
Make `SignService` to unwrap errors and convert them to status message
during writing to the response. Non-status errors are converted to
`INTERNAL` server failures. Status errors can also be wrapped in the
depths of the executable code, so `SignService` tries to unwrap them.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
8a2a096680