added exception handling for the device curl call in the circuitpusher app.

If host din't involve in the network communication, then if we try to add the circuit for that host then it generates the following exception.

```
mininet@mininet-vm:~/floodlight/apps/circuitpusher\> ./circuitpusher.py --controller 127.0.0.1:8080 --type ip --src 10.0.0.1 --dst 10.0.0.3 --add --name cirtuit2
Namespace(action='add', circuitName='cirtuit2', controllerRestIp='127.0.0.1:8080', dstAddress='10.0.0.3', srcAddress='10.0.0.1', type='ip')
curl -s http://127.0.0.1:8080/wm/device/?ipv4=10.0.0.1

Traceback (most recent call last):
  File "./circuitpusher.py", line 97, in <module>
    sourceSwitch = parsedResult[0]['attachmentPoint'][0]['switchDPID']
IndexError: list index out of range
```
So in PR, I have added lines for the exception handling.
Please suggest for any improvement.

Thanks,