extension function call error handling

[Benoit Cerrina <be...@writeme.com>]

Hi,
in a previous message I've reported encountering a NullPointerException when trying to call an extension function I had wrote.  It turned out that the arguments where not correct, however, I believe that the error handling there left a lot to be desired, therefore I trapped this particular exception and rethrew it with a more meaningfull message,  attached is a diff of XSLTJavaClassEngine with the new treatment for the exception.
I couldn't find a way to get the line number of the method call so it still needs to be improved, however it would be nice if one of you could check this change in.
Benoit