Only attempt to derive jp2 copies from tiffs

If you upload a JPEG master you will run into trouble when you try to generate a JPEG2000 access copy.  This method only works with TIFFs, so we can throw an exception if you get something else.

I'm not sure if `IllegalStateException' is the right Exception to throw, however.  I would appreciate any advice on that. ;-)

@scoen, please have a gander.

Huw.

(Note: this is very similar to nla/banjo/pull/311 )