fsck: don't return error if fsck.<type> does not exist

The error message is expected for "really wanted" set of filesystems
(extN, ..), otherwise it does not make sense for filesystems like
btrfs or xfs.

Reported-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: Karel Zak <kzak@redhat.com>