Fix warning on nonexistent quest.lua text

Cavemen don't have goal_alt message - before lua, that one
fell through into goal_next message, but now it tried to
load the "common" message. Add ability to define message fallbacks,
and make goal_next the fallback for goal_alt.

Also prevent issuing quest.lua errors twice.

dat/quest.lua

@@ -4,7 +4,7 @@
 --  - export the quest string replacements to lua, instead of %H etc
 --  - allow checking if hero is carrying item (see comments for %Cp Arc 00042)
 --  - fold quest_portal, quest_portal_again, quest_portal_demand into one
---  - some roles have no goal_alt, fold into goal_next?
+--  - write tests to check questtext validity?
 --  - qt_pager hack(?): if (qt_msg->delivery == 'p' && strcmp(windowprocs.name, "X11"))
 
 
@@ -25,6 +25,10 @@
 
 
 questtext = {
+   -- If a role doesn't have a specific message, try a fallback
+   msg_fallbacks = {
+      goal_alt = "goal_next"
+   },
    common = {
       TEST_PATTERN = {
          output = "text",

src/questpgr.c

@@ -432,9 +432,10 @@ boolean common UNUSED;
 }
 
 boolean
-com_pager_core(section, msgid)
+com_pager_core(section, msgid, showerror)
 const char *section;
 const char *msgid;
+boolean showerror;
 {
     const char *const howtoput[] = { "pline", "window", "text", "menu", "default", NULL };
     const int howtoput2i[] = { 1, 2, 2, 3, 0, 0 };
@@ -442,6 +443,7 @@ const char *msgid;
     lua_State *L;
     char *synopsis;
     char *text;
+    char *fallback_msgid = NULL;
 
     if (skip_pager(TRUE))
         return FALSE;
@@ -449,7 +451,8 @@ const char *msgid;
     L = nhl_init();
 
     if (!nhl_loadlua(L, QTEXT_FILE)) {
-        impossible("com_pager: %s not found.", QTEXT_FILE);
+        if (showerror)
+            impossible("com_pager: %s not found.", QTEXT_FILE);
         lua_close(L);
         return FALSE;
     }
@@ -457,21 +460,40 @@ const char *msgid;
     lua_settop(L, 0);
     lua_getglobal(L, "questtext");
     if (!lua_istable(L, -1)) {
-        impossible("com_pager: questtext in %s is not a lua table", QTEXT_FILE);
+        if (showerror)
+            impossible("com_pager: questtext in %s is not a lua table",
+                       QTEXT_FILE);
         lua_close(L);
         return FALSE;
     }
 
     lua_getfield(L, -1, section);
     if (!lua_istable(L, -1)) {
-        impossible("com_pager: questtext[%s] in %s is not a lua table", section, QTEXT_FILE);
+        if (showerror)
+            impossible("com_pager: questtext[%s] in %s is not a lua table",
+                       section, QTEXT_FILE);
         lua_close(L);
         return FALSE;
     }
 
-    lua_getfield(L, -1, msgid);
+tryagain:
+    lua_getfield(L, -1, fallback_msgid ? fallback_msgid : msgid);
     if (!lua_istable(L, -1)) {
-        impossible("com_pager: questtext[%s][%s] in %s is not a lua table", section, msgid, QTEXT_FILE);
+        if (!fallback_msgid) {
+            /* Do we have questtxt[msg_fallbacks][<msgid>]? */
+            lua_getfield(L, -3, "msg_fallbacks");
+            if (lua_istable(L, -1)) {
+                fallback_msgid = get_table_str_opt(L, msgid, NULL);
+                lua_pop(L, 2);
+                if (fallback_msgid)
+                    goto tryagain;
+            }
+        }
+
+        if (showerror)
+            impossible("com_pager: questtext[%s][%s] in %s is not a lua table",
+                       section, msgid, QTEXT_FILE);
+        free(fallback_msgid);
         lua_close(L);
         return FALSE;
     }
@@ -487,9 +509,11 @@ const char *msgid;
         nelems = (int) lua_tointeger(L, -1);
         lua_pop(L, 1);
         if (nelems < 2) {
-            impossible(
+            if (showerror)
-              "com_pager: questtext[%s][%s] in %s in not an array of strings",
+                impossible(
+                "com_pager: questtext[%s][%s] in %s in not an array of strings",
                        section, msgid, QTEXT_FILE);
+            free(fallback_msgid);
             lua_close(L);
             return FALSE;
         }
@@ -518,6 +542,7 @@ const char *msgid;
         free(synopsis);
     }
 
+    free(fallback_msgid);
     free(text);
     lua_close(L);
     return TRUE;
@@ -527,15 +552,15 @@ void
 com_pager(msgid)
 const char *msgid;
 {
-    com_pager_core("common", msgid);
+    com_pager_core("common", msgid, TRUE);
 }
 
 void
 qt_pager(msgid)
 const char *msgid;
 {
-    if (!com_pager_core(g.urole.filecode, msgid))
+    if (!com_pager_core(g.urole.filecode, msgid, FALSE))
-        com_pager_core("common", msgid);
+        com_pager_core("common", msgid, TRUE);
 }
 
 struct permonst *