Exception testing for CTS mode

[timw]

Add exception testing (as already exist for other modes) for CTS mode.
Fix a couple of minor issues arising from those tests.

Added TODO to fix hacky checking of cipher compatibility when there is a stable mechanism to differentiate primitive block ciphers from block cipher modes - BC isn't very clean about the block cipher/block cipher mode separation.

[bcgit]

It's about as clean as the way we produce the modes... I agree about the TODO though, the conditional will continue to telescope if a marker interface isn't used. Going to ponder it, my feeling at the moment is introducing a BlockCipherEngine interface which extends BlockCipher may be the answer - then the if would be against BlockCipherEngine and CBCBlockCipher. I have left the TODO in place.