improved error messaging when ledger is asleep

[stephengaudet]

Steps to Reproduce

1. Have ledger vault configured with a low value for close_after
2. Start signatory
3. Wait longer than close_after until ledger screen saver activates
4. Query a key: curl <signatory>:6732/keys/<pkh>

Expected

A helpful error message like:

[{"id":"failure","kind":"temporary","msg":"this key not found in any vault. Please unlock the Ledger device; it could not be queried."}]

Actual

% curl localhost:6732/keys/tz2GLxED2Nx5Mtii3dFXhcHy4Rt8cHMBtpYb
[{"id":"failure","kind":"temporary","msg":"(Ledger/): [0x6400]: Unknown error"}]

The 0x6400 error code from the Ledger is not translated into a user-friendly message.