fence_rsb: Raise exceptions not strings
    
    otherwise we'll see in python 2.6
    
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: exceptions must be old-style classes or derived from BaseException, not str
    
    Signed-off-by: Marek 'marx' Grac <mgrac@redhat.com>
    Resolves: rhbz#612941