Qt tx dialog: warn if user asked for full bip32 paths but info missing

related: https://github.com/spesmilo/electrum/issues/5969#issuecomment-591441399

Instead of a log line, maybe it should warn as part of the GUI.. but this is a start.

electrum/gui/qt/transaction_dialog.py

@@ -266,6 +266,14 @@ class BaseTxDialog(QDialog, MessageBoxMixin):
             raise Exception("Can only export partial transactions for hardware device.")
         tx = copy.deepcopy(self.tx)
         tx.add_info_from_wallet(self.wallet, include_xpubs_and_full_paths=True)
+        # log warning if PSBT_*_BIP32_DERIVATION fields cannot be filled with full path due to missing info
+        from electrum.keystore import Xpub
+        def is_ks_missing_info(ks):
+            return (isinstance(ks, Xpub) and (ks.get_root_fingerprint() is None
+                                              or ks.get_derivation_prefix() is None))
+        if any([is_ks_missing_info(ks) for ks in self.wallet.get_keystores()]):
+            _logger.warning('PSBT was requested to be filled with full bip32 paths but '
+                            'some keystores lacked either the derivation prefix or the root fingerprint')
         return tx
 
     def copy_to_clipboard(self, *, tx: Transaction = None):