Qt frontend: Handle 'aborted' prompt replies.

On some rare occasions, the execution request for prompt
is aborted by the kernel (due to an error in another execution request -
https://github.com/ipython/ipython/blob/master/IPython/kernel/zmq/ipkernel.py#L469 ),
which is not handled gracefully by the ipython qt frontend widget
raising `KeyError: 'execution_count'`
```
Traceback (most recent call last):
  File "/home/pankaj/work/ipython/IPython/qt/base_frontend_mixin.py", line 138, in _dispatch
    handler(msg)
  File "/home/pankaj/work/ipython/IPython/qt/console/ipython_widget.py", line 178, in _handle_execute_reply
    number = msg['content']['execution_count'] + 1
KeyError: 'execution_count'
```

This commit simply handles 'aborted' prompt requests by retrying it.