zdtm.py: catch KeyError instead of ValueError for sets

In [3]: a
Out[3]: {1, 2, 3}

In [4]: a.remove(5)
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-4-af76ca25878a> in <module>()
----> 1 a.remove(5)

KeyError: 5

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>