Coroutines do not unwind the stack in case of errors

Back to how it was, a coroutine does not unwind its stack in case of
errors (and therefore do not close its to-be-closed variables). This
allows the stack to be examined after the error. The program can
use 'coroutine.kill' to close the variables.

The function created by 'coroutine.wrap', however, closes the
coroutine's variables in case of errors, as it is impossible to examine
the stack any way.

testes/coroutine.lua

@@ -346,9 +346,13 @@ do
   local st, res = coroutine.resume(B)
   assert(st == true and res == false)
 
-  A = coroutine.wrap(function() return pcall(A, 1) end)
+  local X = false
+  A = coroutine.wrap(function()
+    local *toclose _ = setmetatable({}, {__close = function () X = true end})
+    return pcall(A, 1)
+  end)
   st, res = A()
-  assert(not st and string.find(res, "non%-suspended"))
+  assert(not st and string.find(res, "non%-suspended") and X == true)
 end