Sophisticated Sagitta

This release adds exception handling to the send function, allowing routers to raise an exception preventing messages from being marked as sent if an error occurred during sending.

There is no special handling of these unsent messages, so it is good to keep an eye on the logs to notice when this is happening.

This release also defines exceptions previously defined in the comet-common library. The exceptions in comet-common are now regarded as deprecated and will be replaced with the corresponding exception from core.