The main thread cannot be closed

No thread started with pcall (instead of resume) can be closed,
because coroutine.close would not respect the expected number of
results from the protected call.

testes/coroutine.lua

@@ -158,6 +158,11 @@ do
 
   local main = coroutine.running()
 
+  -- cannot close 'main'
+  local st, msg = pcall(coroutine.close, main);
+  assert(not st and string.find(msg, "main"))
+
+
   -- cannot close a "normal" coroutine
   ;(coroutine.wrap(function ()
     local st, msg = pcall(coroutine.close, main)