Update Cached Convertable interface

- The interface says that getConversionSize raises a KeyError when this cache
  does not exist. Update implementation accordingly.
- update getConversion to always return a string for the data. The caller
  should not have to worry about the data format.
- warn when an OFS.Image is passed to setConversion
deferred_style: make sure that the number of report sections is correct in the final report