Fix 500 on protocol endpoint

[2020-04-16T15:24:56 +0000] | api | ERROR - Unknown error caught in API - 'Pending' object has no attribute 'generate_auction_protocol'
Traceback (most recent call last):
  File "/auction/prozorro_sale/auction/errors.py", line 119, in request_localized_errors_middleware
    return await handler(request)
  File "/usr/local/lib/python3.8/site-packages/prozorro_sale/auth/__init__.py", line 26, in token_middleware
    return await handler(request)
  File "/auction/prozorro_sale/auction/api/auth.py", line 14, in request_context_middleware
    return await handler(request)
  File "/auction/prozorro_sale/auction/api/auth.py", line 8, in bidder_auth
    return await handler(request)
  File "/usr/local/lib/python3.8/site-packages/prozorro_sale/procedure/errors.py", line 116, in retry_on_concurrency_error_middleware
    return await handler(request)
  File "/auction/prozorro_sale/auction/utils.py", line 42, in handler
    return await func(request, *args, **kwargs)
  File "/auction/prozorro_sale/auction/api/api.py", line 197, in generate_protocol
    return web.json_response(auction.generate_auction_protocol(), status=200)
  File "/auction/prozorro_sale/auction/state_machine.py", line 566, in __getattr__
    return getattr(self.get_current_state(), item)
AttributeError: 'Pending' object has no attribute 'generate_auction_protocol'
[2020-04-16T15:24:56 +0000] | aiohttp.access | INFO - "93.76.196.231, 10.198.8.238 GET /api/auctions/5e81d47e30e588eb198a5483/protocol HTTP/1.1 500 341 - Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.0.2 Safari/605.1.15 0.002771"