Allow yields in '__close' metamethods ater errors

Completes commit b07fc10e91. '__close' metamethods can yield even
when they are being called due to an error. '__close' metamethods from
C functions are still not allowed to yield.

testes/locals.lua

@@ -697,34 +697,66 @@ end
  
 
 do
-  -- yielding inside closing metamethods after an error:
-  -- not yet implemented; raises an error
+  -- yielding inside closing metamethods after an error
 
   local co = coroutine.wrap(function ()
 
     local function foo (err)
 
+      local z <close> = func2close(function(_, msg)
+        assert(msg == nil or msg == err + 20)
+        coroutine.yield("z")
+        return 100, 200
+      end)
+
+      local y <close> = func2close(function(_, msg)
+        -- still gets the original error (if any)
+        assert(msg == err or (msg == nil and err == 1))
+        coroutine.yield("y")
+        if err then error(err + 20) end   -- creates or changes the error
+      end)
+
       local x <close> = func2close(function(_, msg)
-        assert(msg == err)
+        assert(msg == err or (msg == nil and err == 1))
         coroutine.yield("x")
         return 100, 200
       end)
 
-      if err then error(err) else return 10, 20 end
+      if err == 10 then error(err) else return 10, 20 end
     end
 
     coroutine.yield(pcall(foo, nil))  -- no error
+    coroutine.yield(pcall(foo, 1))    -- error in __close
     return pcall(foo, 10)     -- 'foo' will raise an error
   end)
 
-  local a, b = co()
+  local a, b = co()   -- first foo: no error
   assert(a == "x" and b == nil)    -- yields inside 'x'; Ok
-
+  a, b = co()
+  assert(a == "y" and b == nil)    -- yields inside 'y'; Ok
+  a, b = co()
+  assert(a == "z" and b == nil)    -- yields inside 'z'; Ok
   local a, b, c = co()
   assert(a and b == 10 and c == 20)   -- returns from 'pcall(foo, nil)'
 
-  local st, msg = co()    -- error yielding after an error
-  assert(not st and string.find(msg, "attempt to yield"))
+  local a, b = co()   -- second foo: error in __close
+  assert(a == "x" and b == nil)    -- yields inside 'x'; Ok
+  a, b = co()
+  assert(a == "y" and b == nil)    -- yields inside 'y'; Ok
+  a, b = co()
+  assert(a == "z" and b == nil)    -- yields inside 'z'; Ok
+  local st, msg = co()             -- reports the error in 'y'
+  assert(not st and msg == 21)
+
+  local a, b = co()    -- third foo: error in function body
+  assert(a == "x" and b == nil)    -- yields inside 'x'; Ok
+  a, b = co()
+  assert(a == "y" and b == nil)    -- yields inside 'y'; Ok
+  a, b = co()
+  assert(a == "z" and b == nil)    -- yields inside 'z'; Ok
+  local st, msg = co()    -- gets final error
+  assert(not st and msg == 10 + 20)
+
 end