`client::get_key()` should allow the caller to distinguish item not found from an actual error.

[ximon18]

From NLnetLabs/cascade-hsm-bridge#32 (comment).

Currently get_key() cannot distinguish, nor can its callers, between item not found and an actual error, both result in an internal error.

If get_key() could return Result<Option< instead that would allow the caller to distinguish the cases.

Alternatively, if the do_request() fn did not hide the actual error (item not found) by mapping it to a generic internal error, the callers would be able to detect item not found as the actual error.

Either way, some improvement is needed here.