Debugger: Properly exit when engine run failed

Instantly stop the engine if e.g. the port is blocked. If we let
it running until the user closes the dialog, the timeout for automatic
connects would be triggered, and we'd get yet another dialog afterwards
telling the user that the connection failed.

Change-Id: Ifd0e9d743f3b8f4ba853be032d9527559ef6004e
Reviewed-by: Aurindam Jana <aurindam.jana@nokia.com>