test_nodelet: test for #23 (display helpful error msg)
nodelet: Loader: do not call impl->refresh_classes_ if not available

This fixes #23. The call on an uninitialized boost::function was causing
an exception, which was reported to the user instead of the underlying
exception from the failed library load.
Merge pull request #24 from xqms/fix-23

nodelet: Do not hide error if a nodelet fails to load