improved error messaging when ledger is asleep

[stephengaudet]

steps to reproduce:

1.have ledger vault configured. use a low value for ledger vault close_after
2.start signatory
3.wait some time longer than close_after and ledger screen saver is on
5.curl :6732/keys/<well_formatted_but_nonexistent_pkh>

expected:
the error message when a key cannot be found is:
[{"id":"failure","kind":"temporary","msg":"this key not found in any vault"}]
we could include with that something to the effect "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"}]