All Games|Menu: Disable selection of episodes whose start map is missing

With the shareware game modes, the "buy the full game prompt" will be
displayed. Otherwise, disable the relevant button on the episode menu
and log a warning for the mod author.

doomsday/plugins/common/src/hu_menu.cpp

@@ -3071,23 +3071,35 @@ void Hu_MenuInitEpisodePage()
             btn->setHelpInfo(helpInfo);
         }
 
-#if __JDOOM__ || __JHERETIC__
         de::Uri startMap(info.gets("startMap"), RC_NULL);
-        if(
-#if __JHERETIC__
-           gameMode == heretic_shareware
-#else // __JDOOM__
-           gameMode == doom_shareware
-#endif
-           && startMap.path() != "E1M1")
+        if(P_MapExists(startMap.compose().toUtf8().constData()))
         {
-            btn->actions[Widget::MNA_ACTIVEOUT].callback = Hu_MenuActivateNotSharewareEpisode;
+            btn->actions[Widget::MNA_ACTIVEOUT].callback = Hu_MenuSelectEpisode;
+            btn->setData(String::fromStdString(it->first));
         }
         else
-#endif
         {
-            btn->actions[Widget::MNA_ACTIVEOUT].callback = Hu_MenuSelectEpisode;
-            btn->setData(String::fromStdString(it->first));
+#if __JDOOM__ || __JHERETIC__
+            // In shareware display a prompt to buy the full game.
+            if(
+#if __JHERETIC__
+               gameMode == heretic_shareware
+#else // __JDOOM__
+               gameMode == doom_shareware
+#endif
+               && startMap.path() != "E1M1")
+            {
+                btn->actions[Widget::MNA_ACTIVEOUT].callback = Hu_MenuActivateNotSharewareEpisode;
+            }
+            else
+#endif
+            {
+                // Disable this selection and log a warning for the mod author.
+                btn->setFlags(FO_SET, MNF_DISABLED);
+                LOG_RES_WARNING("Failed to locate the starting map \"%s\" for episode '%s'."
+                                " This episode will not be selectable from the menu")
+                        << startMap << String::fromStdString(it->first);
+            }
         }
 
         btn->actions[Widget::MNA_FOCUS].callback = Hu_MenuDefaultFocusAction;