CASTOR-4800: failed migration mount of tapeserverd is not propagted to disk IO threads
We had a session ending mechanism for the tape and disk tasks in migrations, but it did not cover all the aspects of the tape thread (like in this ticket, mounting). In this case the disk thread was happily reading the data from disk, which the tape thread was putting to the bin. The signalling mechanism, attached to the task injector has now been passed to tape write thread itself, which signals the error condition when receiving any exception. If this came from a task as before, this is a no-op, but if we had a problem mounting, then the session will and immediately and disk threads will stop reading (also immediately).