Avoid masking handling errors as decoding errors

This came up in #16 where an exception during message handling gets
swallowed and the error is about decoding.

autoload/lsc/protocol.vim

@@ -40,10 +40,10 @@ function! lsc#protocol#consumeMessage(ch_id) abort
   try
     let content = json_decode(payload)
     if type(content) != v:t_dict | throw 1 | endif
-    call lsc#dispatch#message(content)
   catch
     call lsc#util#error('Could not decode message: '.payload)
   endtry
+  if exists('l:content') | call lsc#dispatch#message(content) | endif
   let remaining_message = message[message_end:]
   call lsc#server#setBuffer(a:ch_id, remaining_message)
   if remaining_message != ''