fix(pubsub): check both client errors on Close

[hongalex]

In #2624, we split the publisher and subscriber client's connection pool. Thus, we need to check both errors returned, rather than just the first one.

[shollyman]

Approving, but please see the comment about close semantics and address if necessary.

[xmlking]

@hongalex I was debugging why I am getting "pubsub publisher closing error: rpc error: code = Canceled desc = grpc: the client connection is closing error when called client.Close()
After upgrading to cloud.google.com/go/pubsub v1.8.0 I started see this error. I never got this error in previous pubsub versions.

I accidentally discovered there is a type in the error message at:
https://github.com/googleapis/google-cloud-go/blob/master/pubsub/pubsub.go#L100-L109

this should be:

func (c *Client) Close() error {
	pubErr := c.pubc.Close()
	subErr := c.subc.Close()
	if pubErr != nil {
                 return fmt.Errorf("pubsub publisher closing error: %v", pubErr)
	}
	if subErr != nil {
		return fmt.Errorf("pubsub subscriber closing error: %v", subErr)
	}
	return nil
}

[xmlking]

Should we detect PubSub emulator environment and automatically suppress the client connection is closing error ?
otherwise this leads to confusion to users

[hongalex]

Hey @xmlking, this error does occur with the emulator and the pstest fake server. I'll address this in a separate PR so we can suppress "the client connection is closing" errors. Also, thanks for catching the typo.

Btw, I noticed in the linked issue, there's a comment that says

Live pubsub server will throw this error

Though I think the opposite is true (live pub/sub service will not throw this error).

[xmlking]

thanks @hongalex . fixed my type