concurrent: Add future_set_exception_unless_cancelled

concurrent: Add future_set_exception_unless_cancelled

Tornado's limited support for cancellation means that errors after a
Future is cancelled could get raised (and probably logged) as
InvalidStateErrors.

This new function effectively changes the behavior to log the real
exception instead of the InvalidStateError. We log them instead of
ignoring them because it's generally not a good idea to let errors
pass silently even if they're after the point that no one is listening
for them (this is consistent with the way gen.multi() handles multiple
errors, for example).

Fixes #2540