hey you're one of those thinking: " hey let others do the work" and don't give further infos
At first edit your post and delete the error message - it contains personal informations!
so after my brain worked a bit i come to the conclusion you tried the default client (Play Button at top)
As the server has some lags it might come to hickups too and can't DL some needed stuff in apropriate time so you'll get the error.
(your internet connection might come into play too) - i checked the missing file is downlodable...
Now you can try again or better get the Ender client for your system
viewtopic.php?f=27&t=14575important: Ender 32 Bit with 32 Bit Java only or Ender 64 Bit with Java 64 Bit!
also DL the update.zip and unpack into Ender folder