Fix the following bugs in the C implementation:

* get_future_loop() silenced all exceptions raised when look up the get_loop
  attribute, not just an AttributeError.
* enter_task() silenced all exceptions raised when look up the current task,
  not just a KeyError.
* repr() was called for a borrowed link in enter_task() and task_step_impl().
* str() was used instead of repr() in formatting one error message (in
  Python implementation too).
* There where few reference leaks in error cases.