ExpiredToken errors should be thrown when calling postActivity about botframework-directlinejs

[billba]

Most posting errors are nonrecoverable (reposting is not expected to yield different results), and in those cases we throw an error. But if a token is expired a new one can be obtained, and the message can be reposted. So instead of throwing an error here, we mark the message as "retry" so that it can be reposted later. This admittedly an inelegant solution. It's convenient for WebChat, but maybe not so much for other applications.

The way to handle these is to subscribe to connectionStatus$ and handle ConnectionStatus.ExpiredToken.