If an incoming call is terminated before we answer (or indeed after we answer), the group change reason is None. We don't have an error that corresponds well to this.
o.fd.T.Errors.Terminated: channel terminated for an unspecified reason (and we can use that for termination by peer where it makes sense)
Spec meeting approved the above suggestion; I will write a patch and get it insta-reviewed.
Fixed in git and reviewed by wjt, will be in 0.17.23.
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