Handle error if node is unavailable

[Bill Hsu]

For example, hash_bundle = "" will be returned when there is an error occurred during the interaction with the node, and it can lead to further error.

SCoin/app/did.py

Line 36 in bc19a0e

 hash_bundle = send_transfer(data, receiver_address) 

[HuangJyunYu]

The status of public-chain changeable, you could use a async-queue to cache the transactions and process any troubleshooting. Refer the code segment for detail: link