Better error messages for some polymorphic functions

New auxiliary functions/macros 'luaL_argexpected'/'luaL_typeerror'
ease the creation of error messages such as

  bad argument #2 to 'setmetatable' (nil or table expected, got boolean)

(The novelty being the "got boolean" part...)

ldblib.c

@@ -55,8 +55,7 @@ static int db_getmetatable (lua_State *L) {
 
 static int db_setmetatable (lua_State *L) {
   int t = lua_type(L, 2);
-  luaL_argcheck(L, t == LUA_TNIL || t == LUA_TTABLE, 2,
-                    "nil or table expected");
+  luaL_argexpected(L, t == LUA_TNIL || t == LUA_TTABLE, 2, "nil or table");
   lua_settop(L, 2);
   lua_setmetatable(L, 1);
   return 1;  /* return 1st argument */