Added a StructureNotReadyException that implementers can throw when they haven't been configured sufficiently in order to read the structure (or data).

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka/trunk@9866 e0a1b77d-ad91-4216-81b1-defd5f83fa92
Now checks for the new StructureNotReadyException so that a warning can be printed to the log/status area (rather than just an exception getting dumped to the console) in the situation where the base Loader has deemed that it is not configured correctly and the structure cannot be provided.

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka/trunk@9867 e0a1b77d-ad91-4216-81b1-defd5f83fa92
Merge remote-tracking branch 'upstream'