Handle unsubscribe pending confirmation

[markdboyd]

Changes proposed in this pull request:

• Update unsubscribe_notification_topic to not throw error if the subscription is still pending confirmation and cannot be unsubscribed. This will fix failing integration tests where the subscription is never actually confirmed while the tests are running

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

None, just fixing broker behavior