WIP: Handle exceptions in job.child_complete() properly

To demonstrate an exception, make a project.conf file with a name: element with a space in it, e.g. name: BSD Games, then try to track it with bst track <element>. At the moment, this causes an unhandled exception and backtrace and bst continues waiting. This patch copies the exception handling code from Job.child_action to Job.child_complete. I've had to remove some arguments as the timing information and filename aren't available there.