removed all uses of undefined from code base

It's a code smell, can lead to hard to diagnose error messages.

Crypto.hs

@@ -93,7 +93,7 @@ genSharedCipher highQuality =
 {- Updates an existing Cipher, re-encrypting it to add or remove keyids,
  - depending on whether the first component is True or False. -}
 updateEncryptedCipher :: [(Bool, String)] -> StorableCipher -> IO StorableCipher
-updateEncryptedCipher _ SharedCipher{} = undefined
+updateEncryptedCipher _ SharedCipher{} = error "Cannot update shared cipher"
 updateEncryptedCipher [] encipher = return encipher
 updateEncryptedCipher newkeys encipher@(EncryptedCipher _ variant (KeyIds ks)) = do
 	dropKeys <- listKeyIds [ k | (False, k) <- newkeys ]