errors and exceptions in xproc

A few thoughts on error processing in xproc:

* has it been considered that try/catch throws exceptions and
everything else errors, I like c:exception better versus c:error

* should a p:error have a "error" output port

* I was thinking of how nice it would be to output all error messages
as well formed documents ... perhaps this could be achieved using a
top level try/catch but I guess there is scope for this type of
functionality as an extension attribute on p:pipeline and leave it to
the implementator to provide

cheers, Jim Fuller