Sulley close on "Restarting the target failed, exiting" when target is restarted by process_monitor.py

When the target crashed, process_monitor restarted the target (function start_target is invoked).

Return value of start_target is used by session.py at line 772. 
If None is returned, restart_target function fails and Sulley closes. (line 700-701).

By adding a return value for start_target in process_monitor.py, It indicates to session.py that the process has been sucessfully restarted and the fuzzing session can continue.