HorizontallyAlignedFlowable: make more robust
HorizontallyAlignedFlowable: make more robust

- if an exception other than EndOfContainer was raised, width would not
  be defined when reaching the finally clause (and the original
  exception would be shadowed by the NameError exception)
- if the image file was not found the EndOfContainer exception could
  have been raised while rendering the warning paragraph, who's state
  does not have a width attibute

brechtm/rinohtype,brechtm/rinohtype,brechtm/rinohtype