[Qt] Show more significant warning if we fall back to the default fee

src/qt/forms/sendcoinsdialog.ui

@@ -760,10 +760,30 @@
            </layout>
           </item>
           <item>
+           <widget class="QLabel" name="fallbackFeeWarningLabel">
+            <property name="toolTip">
+             <string>Using the fallbackfee can result in sending a transaction that will take serval hours or days (or never) to confirm. Consider choosing your fee manually or wait until your have validated the complete chain.</string>
+            </property>
+            <property name="styleSheet">
+             <string notr="true">color: rgb(255, 150, 0);
+font-weight: bold;</string>
+            </property>
+            <property name="text">
+             <string>Warning: Fee estimation is currently not possible.</string>
+            </property>
+            <property name="wordWrap">
+             <bool>false</bool>
+            </property>
+           </widget>
+          </item>
+          <item>
            <spacer name="horizontalSpacer_4">
             <property name="orientation">
              <enum>Qt::Horizontal</enum>
             </property>
+            <property name="sizeType">
+             <enum>QSizePolicy::MinimumExpanding</enum>
+            </property>
             <property name="sizeHint" stdset="0">
              <size>
               <width>40</width>

src/qt/sendcoinsdialog.cpp

@@ -655,13 +655,15 @@ void SendCoinsDialog::updateSmartFeeLabel()
                                                                 std::max(CWallet::fallbackFee.GetFeePerK(), CWallet::GetRequiredFee(1000))) + "/kB");
         ui->labelSmartFee2->show(); // (Smart fee not initialized yet. This usually takes a few blocks...)
         ui->labelFeeEstimation->setText("");
+        ui->fallbackFeeWarningLabel->setVisible(true);
     }
     else
     {
         ui->labelSmartFee->setText(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(),
                                                                 std::max(feeRate.GetFeePerK(), CWallet::GetRequiredFee(1000))) + "/kB");
         ui->labelSmartFee2->hide();
         ui->labelFeeEstimation->setText(tr("Estimated to begin confirmation within %n block(s).", "", estimateFoundAtBlocks));
+        ui->fallbackFeeWarningLabel->setVisible(false);
     }
 
     updateFeeMinimizedLabel();