Ensure Etherscan result is valid before reading it

[whymarrh]

This PR updates the conditions under which we attempt to read the results of an incoming tx list from Etherscan. We were previously reading the result when the status was non-zero but that saw type errors where the result property was null or a string.

We now handle the following responses:

{"status":"0","message":"No transactions found","result":[]}

{"status":"1","message":"OK","result":null}

{"status":"0","message":"NOTOK","result":"Error! Missing Or invalid Action name"}

[Gudahtt]

Looks good!