Server api logic about alpaca_* such as alpaca_online_learning alpaca_active_learning and so on

[svjack]

the code seems have many

response = alpaca_client.online_learning(train_docs, annotations, epoch, batch
 if response == 'error':
        print('error')
        time.sleep(2)
        alpaca_client.online_learning(train_docs, annotations, epoch, batch)

if the first line return "error" it is more possiable always return "error" after sleep 2s
which will always yield status code 500 of response.
For example, some score of rnn decode may be inf.
This may occur when the downstream tag fitting process not stable (as normal training of
some neutral network)
Users should have a monitor of this incremental training process. If the network fitted bad should
Terminated before going wrong way too long.
Only return status 500 with frontend blocked seems not appropriate.

So i think should add a realtime validation interface by client.