Publisher Client does not enter paused state before future.set_exception

[mukund-ananthu]

For non-retriable errors returned from the backend, the client library currently sets future.set_exception() for all outstanding futures:

python-pubsub/google/cloud/pubsub_v1/publisher/_batch/thread.py

Lines 285 to 286 in 8c7e2a9

	for future in self._futures:
	future.set_exception(exc)

before it tries to change its state to paused by calling the _batch_done_callback():

python-pubsub/google/cloud/pubsub_v1/publisher/_batch/thread.py

Lines 289 to 291 in 8c7e2a9

	if self._batch_done_callback is not None:
	# Failed to publish batch.
	self._batch_done_callback(batch_transport_succeeded)

This results in race conditions where the callback associated with the future is invoked before the client enters paused state. Which causes issues if the callback attempts to do a resume_publish() before the client is in the paused state.