python: Use error description in SanlockException
    
    When raising SanlockException, if the error code was a posix error, we
    included the error description from strerror(3), but for sanlock error
    codes we included a rather unhelpful "Sanlock exception".
    
    Now that we have sanlock_strerror() we can use it to include the actual
    error description:
    
        sanlock.SanlockException: (-223, 'Sanlock resource read failure',
        'Lease does not exist on storage')
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>