Reviewed various exception handlings in runtime.
Reviewed various exception handlings in runtime.

Added more comments. Removed unnecessary code. Removed
ShKeyboardInterrupt as KeyboardInterrupt can now be handled properly.
Removed ShCompleteCommand.

ywangd/stash,cclauss/stash,ywangd/stash,jsbain/stash,cclauss/stash,jsbain/stash