Add Flow.onErrorComplete operator

[He-Pin]

When we encounter an exception, sometime we may don't want the flow to result in an error but completes normally.

The implementation can be simple, but still cause some confusing.

I would like suggest to add a onErrorComplete operator which will complete the flow on some dedicated condition.

And better we have another onErrorContiune operator, which will keep the flow working but will need the operator before it to support this.

[pjfanning]

Moved to #624 and otherwise, there is little point separating this discussion from #605