Extend MockAgent so it can simulate msg rejection on Apple side (err 8)

Other causes of rejected message should be prevented on `apnagent` side - such as serialisation problems, but `BAD_TOKEN` error is the only one that may cause the message to be rejected.

Having this is important to properly test error mitigation.
Merge pull request #6 from lemonlabs/ft/mockagent

Extend MockAgent so it can simulate msg rejection on Apple side (err 8)