[Fixed] Minor bug encountered during voting for a poll

**Steps to reproduce the bug**

1. Open any poll .
2. Without selecting any option , click vote.

**Error** : "Key 'choice' not found in \<QueryDict: {u'csrfmiddlewaretoken': [u'xxxxxxxxxxx.....xxx']}>"

**Expected outcome** : Some error message that notifies the user to select a choice should be thrown and then user should be allowed to leave the page ( after voting, that is).

**Fix** : The user stays on the same page  if he doesn't select a valid option , with an error message that reads "You didn't select any choice" .