Keys are matched as bare prefixes, so `Caused by:` inside an abort message
reaches the `Cause` key, fails the key comparison and raises — and the
per-line loop turns that into an error that drops the entire text
tombstone, stack trace included.
A key mismatch means "this line is not that key", not "this file is
broken": decline the line and let the remaining keys have their turn. A
line with no colon is declined the same way instead of raising on the
unpack. The same trap has a second form in the field, HiSilicon/Huawei
tombstones printing `code around pc:` against the `code` key.
