README.md: improve error handling in the sample (#16)
README.md: improve error handling in the sample (#16)

When bitcoin RPC throws an error, it fills the result like this:

```
{ result: null,
  error: 
   { code: -12,
     message: 'Error: Keypool ran out, please call keypoolrefill first' },
  id: '1' }
```

Therefore, if result.error is not null, let's print result.error.message and result.error.code, otherwise we don't get any useful info in the log.
drazisil/node-bitcoin-rpc