Fixes incorrect async exception handling. Adds sleepAsync.

The tasyncexceptions test has been added which tests for this incorrect
exception handling behaviour. The problem was that the exception was
raised inside a callback which was called from a previously finished async
procedure. This caused a "Future already finished" error. The fix was to
simply reraise the exception if the retFutureSym is already finished.

sleepAsync was added to help with the reproduction of this test. It should
also be useful for users however.

Finally some debug information was added to futures to help with future
bugs.