Improve handling of kde_projects.xml database.

The exceptions we throw need to be obtained via make_exception otherwise
a horrible-looking Perl error gets developed and I don't feel like
Googling again which exact permutation of "blessed" or "ref" or what
must be used to get what I want.

In addition this is one of the very few commands which might be run in
--pretend mode, and the only one that is not strictly read-only. However
we need some form of information regarding kde_projects.xml if the
method in question is being called at all so I choose to hit the network
instead of having no information in the --pretend output.