qt: Replace stray tfm::format to cerr with qWarning

GUI warnings should go to the log, not to the console (which may not be
connected at all).

src/qt/sendcoinsdialog.cpp

@@ -468,7 +468,7 @@ bool SendCoinsDialog::signWithExternalSigner(PartiallySignedTransaction& psbtx,
         return false;
     }
     if (err) {
-        tfm::format(std::cerr, "Failed to sign PSBT");
+        qWarning() << "Failed to sign PSBT";
         processSendCoinsReturn(WalletModel::TransactionCreationFailed);
         return false;
     }