bug: Handle Ledger error 0x6804 (#87)

If the Ledger is locked, don't panic, return an error instead.

Though this code doesn't appear to be officially documented anywhere,
I've seen it referred to as "unlock device error", and it consistently
appears while the ledger is locked.

ledger/src/common.rs

@@ -187,6 +187,8 @@ pub enum APDUResponseCodes {
     ExecutionError = 0x6400,
     /// WrongLength
     WrongLength = 0x6700,
+    /// UnlockDeviceError
+    UnlockDeviceError = 0x6804,
     /// EmptyBuffer
     EmptyBuffer = 0x6982,
     /// OutputBufferTooSmall
@@ -231,6 +233,9 @@ impl APDUResponseCodes {
                 "[APDU_CODE_EXECUTION_ERROR] No information given (NV-Ram not changed)"
             }
             APDUResponseCodes::WrongLength => "[APDU_CODE_WRONG_LENGTH] Wrong length",
+            APDUResponseCodes::UnlockDeviceError => {
+                "[APDU_CODE_UNLOCK_DEVICE_ERROR] Device is locked"
+            }
             APDUResponseCodes::EmptyBuffer => "[APDU_CODE_EMPTY_BUFFER]",
             APDUResponseCodes::OutputBufferTooSmall => "[APDU_CODE_OUTPUT_BUFFER_TOO_SMALL]",
             APDUResponseCodes::DataInvalid => {
@@ -264,6 +269,7 @@ impl From<u16> for APDUResponseCodes {
             0x9000 => APDUResponseCodes::NoError,
             0x6400 => APDUResponseCodes::ExecutionError,
             0x6700 => APDUResponseCodes::WrongLength,
+            0x6804 => APDUResponseCodes::UnlockDeviceError,
             0x6982 => APDUResponseCodes::EmptyBuffer,
             0x6983 => APDUResponseCodes::OutputBufferTooSmall,
             0x6984 => APDUResponseCodes::DataInvalid,