Transport `send_envelope` should return a `Result`

I would like to create an offline transport that queues envelopes on disk if no connection is available. The only way to currently do this is to copy one of the existing transports and add the offline queueing. This is far from ideal because this either needs to be implemented for every one of the existing transports and keep up with any upstream changes or limit the underlying transports the the offline mode supports.

If `send_envelope` returned a result, it would be possible to wrap any of the existing transports with an offline transport. The client could either ignore the result or errors could be logged at that point rather than currently in every transport.