Display an inline error message when a screenshot fails

Currently when this happens, the GUI does not inform the user.

src/Gui/KSMainWindow.cpp

@@ -487,6 +487,12 @@ void KSMainWindow::imageCopied()
                       KMessageWidget::Information);
 }
 
+void KSMainWindow::screenshotFailed()
+{
+    showInlineMessage(i18n("Could not take a screenshot. Please report this bug here: <a href=\"https://bugs.kde.org/enter_bug.cgi?product=Spectacle\">create a spectacle bug</a>"),
+                      KMessageWidget::Warning);
+}
+
 void KSMainWindow::showPreferencesDialog()
 {
     if (KConfigDialog::showDialog(QStringLiteral("settings"))) {

src/Gui/KSMainWindow.h

@@ -86,6 +86,7 @@ class KSMainWindow: public QDialog
     void setScreenshotAndShow(const QPixmap &pixmap);
     void imageSaved(const QUrl &location);
     void imageSavedAndCopied(const QUrl &location);
+    void screenshotFailed();
 
     Q_SIGNALS:
 

src/SpectacleCore.cpp

@@ -370,6 +370,7 @@ void SpectacleCore::screenshotFailed()
         emit allDone();
         return;
     case StartMode::Gui:
+        mMainWindow->screenshotFailed();
         mMainWindow->setScreenshotAndShow(QPixmap());
     }
 }