http: check if index is present

I had an error, while running the server:

Traceback (most recent call last):
  File "/usr/lib/python2.7/SocketServer.py", line 593, in process_request_thread
    self.finish_request(request, client_address)
  File "/home/benoit/projs/Mailpile/venv/local/lib/python2.7/site-packages/mailpile/httpd.py", line 281, in finish_request
    SimpleXMLRPCServer.finish_request(self, request, client_address)
  File "/usr/lib/python2.7/SocketServer.py", line 334, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/lib/python2.7/SocketServer.py", line 649, in __init__
    self.handle()
  File "/usr/lib/python2.7/BaseHTTPServer.py", line 340, in handle
    self.handle_one_request()
  File "/usr/lib/python2.7/BaseHTTPServer.py", line 328, in handle_one_request
    method()
  File "/home/benoit/projs/Mailpile/venv/local/lib/python2.7/site-packages/mailpile/httpd.py", line 249, in do_GET
    'mailpile_size': len(session.config.index.INDEX)