cgroups migration test: catch error when migrating task

The PID of the task that is not migrated need to be found
by looking into the cgroups file and filtering these information.
If something goes wrong it may happen that the task PID is not
found. In this case an error is raised.

Signed-off-by: Elieva Pignat <Elieva.Pignat@arm.com>
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>