detect when trying to sign with a hw wallet offline in a not supported config

closes #4229

plugins/ledger/ledger.py

@@ -350,6 +350,9 @@ class Ledger_KeyStore(Hardware_KeyStore):
                 self.give_error("No matching x_key for sign_transaction") # should never happen
 
             redeemScript = Transaction.get_preimage_script(txin)
+            if txin.get('prev_tx') is None:  # and not Transaction.is_segwit_input(txin):
+                # note: offline signing does not work atm even with segwit inputs for ledger
+                raise Exception(_('Offline signing with {} is not supported.').format(self.device))
             inputs.append([txin['prev_tx'].raw, txin['prevout_n'], redeemScript, txin['prevout_hash'], signingPos, txin.get('sequence', 0xffffffff - 1) ])
             inputsPaths.append(hwAddress)
             pubKeys.append(pubkeys)