Fix "NotFound" error in _clear_stacks()

_clear_stacks() will raise "NotFound" exception if stack was
deleted before wait its status. This patch adds a try block
to fix the problem.

Change-Id: I29d4e443e5f355dbba188ccebe930f9401f3ccea
Closes-Bug: #1355666
Merge "Fix "NotFound" error in _clear_stacks()"