Fix: if bootstrap failed, it could end with an empty screen instead of error

There are various of ways bootstrap can fail:
- Failing network connection
- Incomplete download
- No write permissions
- Disk full
- (others I forgot)

They all result in a screen with no windows. To ensure we at least
always show something when anything bad happens, if the bootstrap
is not successful, show a screen what the next step for the human
should be.

src/widgets/bootstrap_widget.h

@@ -15,6 +15,13 @@ enum BootstrapBackgroundWidgets {
 	WID_BB_BACKGROUND, ///< Background of the window.
 };
 
+/** Widgets of the #BootstrapErrmsgWindow class. */
+enum BootstrapErrorMessageWidgets {
+	WID_BEM_CAPTION, ///< Caption of the window.
+	WID_BEM_MESSAGE, ///< Error message.
+	WID_BEM_QUIT,    ///< Quit button.
+};
+
 /** Widgets of the #BootstrapContentDownloadStatusWindow class. */
 enum BootstrapAskForDownloadWidgets {
 	WID_BAFD_QUESTION, ///< The question whether to download.