Exception hierarchy.

[Finn Bock]

Hi Team,
I would like to make some changes to the exceptions used in tree building.
First, I dislike throwing SAXParseException from within FOP. In my 
opinion SAXParseException are for reporting problems during the parsing 
of XML, not for reporting subsequent problems with the FO tree.

Second, the exceptions used in FOP should form a structure where 
concrete subclass exception are thrown and a common superclass can be 
used for catching all the different exceptions.

My proposal for the exception hierarchy look like this:
- SAXException
  - FOPException
    - ValidateException
    - PropertyException
Putting all our exceptions into one hierarchy that inherit from 
SAXException helps reduce the number of wrapped exception.

I've uploaded a patch which changes the FO tree building and property 
code. The benefit of the patch is the reduced number of blind catches:

    } catch (XXXException e) {
        // log.error(e);
    }
Any objections?
regards,
finn