Decide how to handle unknown PubSub topics for push notifications

[jskeet]

Currently we use a resource name of projects/unknown-project!/topics/unknown-topic! when we don't know the topic resource name from a PubSub push notification. (If the path on the incoming HTTP request looks like a topic name, we'll use that, so users can set it up in a useful way, but they may well not do so.)

This is inconsistent with other Functions Frameworks.

Options:

• Change to be consistent with at least one of the other languages before the next release
• Decide consciously to go with what we've got and release, and don't change it
• Stick with what we've got at the moment, but document that the format may change in future releases