Set exit code on script errors....
Set exit code on script errors.
Only handle the issue related to .py scripts.
1. interactiveshell: The method safe_execfile is not totally safe now. Errors of user scripts are raised to a higher level, where we can decide what to do.
2. execution: The %run magic catches exceptions from safe_execfile.
3. shellapp: The _run_cmd_line_code method catches exceptions from safe_execfile and sets the right exit status.
Partial-revert:
1. Change execution.py back.
2. Revert some changes in safe_execfile, then modify the excetion
handling logic.
