gui: create tx, launch error dialog if backend throws runtime_error

only will ever happen if something unexpected happened.

src/qt/walletmodel.cpp

@@ -212,7 +212,7 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
         return AmountExceedsBalance;
     }
 
-    {
+    try {
         CAmount nFeeRequired = 0;
         int nChangePosRet = -1;
 
@@ -240,6 +240,11 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
         if (nFeeRequired > m_wallet->getDefaultMaxTxFee()) {
             return AbsurdFee;
         }
+    } catch (const std::runtime_error& err) {
+        // Something unexpected happened, instruct user to report this bug.
+        Q_EMIT message(tr("Send Coins"), QString::fromStdString(err.what()),
+                       CClientUIInterface::MSG_ERROR);
+        return TransactionCreationFailed;
     }
 
     return SendCoinsReturn(OK);