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), which is not handled gracefully by the ipython frontend
raising `KeyError: 'execution_count'`
This commit simply handles 'aborted' prompt requests by retrying it.
Merge pull request #4800 from pankajp/handle-aborted-prompt-reply

Qt frontend: Handle 'aborted' prompt replies.