Send statements to own backing group first (#2927)

* Factor out runtime module into utils.

* First fatal error design.

* Better error handling infra.

* Error handling cleanup.

* Send to peers of our group first.

* Finish backing group prioritization.

* Little cleanup.

* More cleanup.

* Forgot to checkin error.rs.

* Notes.

* Runtime -> RuntimeInfo

* qed in debug assert.

* PolkaErr -> Fault.