cephfs: return internal server error
cephfs: return internal server error

if it is an error from the IsMountPoint
function and the error is not IsNotExist return
it as a internal server error.

Signed-off-by: Madhu Rajanna <60fbf37a92c2a0908e31296d18dadbec5a4dae17@gmail.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi