Merge #868: Replace stray tfm::format to cerr with qWarning

edd4656 qt: Replace stray tfm::format to cerr with qWarning (laanwj)

Pull request description:

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

ACKs for top commit:
  hebasto:
    ACK edd4656, I have reviewed the code and it looks OK.

Tree-SHA512: 32944e00dae0c62bb23e3d7abd486b63e445702483ca03c74c3057ef942f06e771d4d3d3a58fd728582889d6b638fae11ecc536a25febfd89a28522b7d6d08ba

Author: hebasto

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;
     }