failed to download pretrained weights
## 🐛 Bug

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Model I am using (Bert, XLNet....):BERT

Language I am using the model on (English, Chinese....):English

During downloading  pretrained weights with code of modeling_bert.BertForMaskedLM.from_pretrained('bert-base-uncased'),another exception occurred:
![image](https://user-images.githubusercontent.com/52790610/67380476-faafe900-f5bc-11e9-84a1-bd0c8eeb7515.png)



Hi, this seems to be a network error. Are you sure you have access to the internet on this machine, or is it behind a firewall?<|||||>I had exactly the same problem Yesterday and s3.amazonaws.com just was not reachable. We also had the same problem with another service as well. After trying for some time it just started working again.