If a node has to fail a channel but knows that its latest commitment transaction is outdated it should not be required to send it but rather wait for the peer to unilaterally close the channel. 

The proposed solution is not so clean because it might produce a deadlock in which two peers assume they have outdated state and send `error` back and forth without actually force closing. Maybe in such a scenario we could create a protocol that mutually closes with split balance? 

Also replaced the word use with broadcast as it seems more accurate.

It was decided in the spec meeting to combine lightning#932 and lightning#942 as they are
closely related. It was also decided to add a rationale for lightning#932 to clearly
state why nodes must wait for an error before force-closing instead of
eagerly force-closing when detecting that their peer is behind.