screen: a batch Automerge refuses names its changes

Review section 7: Automerge takes the changes judged in a batch all or
none, so one it won't take (a fork of an actor's changes, under a
policy without signatures) fails the whole batch, and every batch
carrying it. The log said who sent the message (`Failed to process sync
message`, `Delta download failed`, with the peer) but not what failed.

The error now names the changes it was given, the first eight, each
with its seq and the node it says made it, so whoever sends the
offending one shows in the log beside what it sent.

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