Problem: Message kind logic for emitting an error message is convoluted
and still results in emitting an unfinished message earlier than
wanted.
Solution: Ensure emsg_multiline() always sets the kind wanted by the caller
and doesn't isn't unset to logic for emitting the source message.
Caller is responsible for making sure multiple message chunks are
not emitted as multiple events by setting `msg_ext_skip_flush`...
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