shut up freezer_state

shut up freezer_state

Not being able to get freezer state is perfectly valid - if the
container does not exist.  The old version of freezer_state
only reported an error on actually reading the cgroup file,
but not on not finding a cgroup file.  Leave it to the caller
to report the error if it is important, since we don't actually
know any useful info here anyway.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>