Fix os.outputof for lua 5.3.4

src/base/os.lua

@@ -460,17 +460,17 @@
 
 		local pipe = io.popen(cmd .. " 2>&1")
 		local result = pipe:read('*a')
-		local b, exitcode = pipe:close()
-		if not b then
-			exitcode = -1
-		end
-
+		local success, what, code = pipe:close()
+		if success then
 			-- chomp trailing newlines
 			if result then
 				result = string.gsub(result, "[\r\n]+$", "")
 			end
 
-		return result, exitcode
+			return result, code, what
+		else
+			return nil, code, what
+		end
 	end