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...)

manual/manual.of

@@ -4979,6 +4979,19 @@ This function never returns.
 
 }
 
+@APIEntry{
+void luaL_argexpected (lua_State *L,
+                       int cond,
+                       int arg,
+                       const char *tname);|
+@apii{0,0,v}
+
+Checks whether @id{cond} is true.
+If it is not, raises an error about the type of the argument @id{arg}
+with a standard message @seeF{luaL_typeerror}.
+
+}
+
 @APIEntry{typedef struct luaL_Buffer luaL_Buffer;|
 
 Type for a @def{string buffer}.
@@ -5713,6 +5726,19 @@ to start the traceback.
 
 }
 
+@APIEntry{const char *luaL_typeerror (lua_State *L,
+                                      int arg,
+                                      const char *tname);|
+@apii{0,0,v}
+
+Raises a type error for argument @id{arg}
+of the @N{C function} that called it,
+using a standard message;
+@id{tname} is a @Q{name} for the expected type.
+This function never returns.
+
+}
+
 @APIEntry{const char *luaL_typename (lua_State *L, int index);|
 @apii{0,0,-}