scheduler: Fix an issue with wrappertasks and exceptions

scheduler: Fix an issue with wrappertasks and exceptions

Using a 'for' loop on a generator is incompatible with calling throw() on
it, because the latter causes the generator to advance to the next yield.
Consequently, ignoring an exception from a subtask in the parent task would
cause the next subtask to be skipped.

This issue does not affect any existing code to date.

The inner and outer loops should each be equivalent to the definition of
"yield from" (http://www.python.org/dev/peps/pep-0380/#formal-semantics) in
PEP 380, with the following simplifications:
 - Assume that the throw() and close() methods exist
 - Don't support send()
 - Don't support return values (these are Python 3-only feature)

Change-Id: Ie29e68d4505f667f408b67b365c37511f73372fe