Add top level Gateway conditions when Routes fail to attach

[robscott]

What would you like to be added:
Top level Gateway conditions that can be used to indicate when a Route has not been attached to any listeners within the Gateway.

Why this is needed:
The alternative of adding this condition to each listener is rather unwieldy.

This idea came from a comment thread with @howardjohn and @youngnick.

[youngnick]

This feels like a separate Condition for sure, that could be effectively treated as a warning, basically saying "a Route tried to attach, but wasn't allowed". I can see an argument both for including (it is useful info that's not otherwise obtainable without querying the implementation somehow), and not including (it could get unwieldy/cause scale problems if there are a lot of Routes).