PubSub: Propagate subscribe callback errors to main thread

[plamut]

Closes #7917.

This PR implements a user request to propagate exceptions from streaming pull callbacks to the main thread awaiting the result of the future returned by the subscriber.subscribe() call.

How to test

Steps to reproduce:

• Prerequsite: Have a working service account, pubsub topic, and subscription created.
• Publish a message to the topic.
• Run the sample subscriber code posted under the issue (adjust subscription name as necessary).

Actual result (before the fix):

• Nothing happens in the main thread. The exception is raised in the callback, but the main thread keeps waiting for for fut.result().

Expected result (after the fix):

• An exception from the callback is propagated to fut.result(), and the main thread exists almost immediately after the callback is invoked.

[plamut]

FWIW, this method signature change is safe, as StreamingPullManager class is only used internally in the subscriber client's subscribe() method, and is not mentioned anywhere in the user docs, at least from what I can see.