tidy: some minor improvements.

- Tweak some comments.
- No need to do the `concat!` trick on `START_MARKER` because it's
  immediately followed by `END_MARKER`.
- Fix an off-by-one error in the line number for an error message.
- When a second start marker is found without an intervening end marker,
  after giving an error, treat it as though it ends the section. It's
  hard to know exactly what to do in this case, but it makes unit
  testing this case a little simpler (in the next commit).
- If an end marker occurs without a preceding start marker, issue an
  error.

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