If you run an Ant target and afterwards, when it is scanning the project for
newly defined tasks, one of them throws e.g. NoClassDefFoundError while
analyzing its task structure, this will be reported with an error dialog. (The
situation can happen e.g. after running real-clean target on nbbuild/build.xml,
where custom NB tasks are compiled, loaded, run, and then later in execution the
classfiles are deleted--after execution is complete, the project may have some
tasks defined which can no longer be resolved.) Such an error should be handled
the same as errors encountered when scanning built-in tasks at startup: warning
exception printed to log file, as they are harmless.

Fix prepared but not tested in trunk.