API: Replace try-except block with @traceback

waitForPid thread was using an explicit try except block for
reporting excpetions.
The patch replaces it with the @traceback decorator,
ensuring that exceptions are logged and simplifying the
code.

Change-Id: Ib9c618bd52f276d23c7f90742f690d20f45a6639
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22813
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>