emit_event now swallows and reports any failure, including crashes in the
built-in default_emit formatting itself; log_change routes through it.
logfmt chain callbacks that raise are logged and treated as fall-through,
and logmigr callbacks get on_error reporting like fatal_error handlers,
so a broken logging callback can no longer abort a transaction or open.
Demo: raw user ids in v0 logs, logfmt-resolved names in v1 logs.
Leo Vasanko
54d01f1e74