Acknowledge 'sentence unclear' feedback instead of closing silently

[sidprasad]

What changed

When a student reports an english-to-LTL sentence as confusing or unclear, the modal previously just closed on submit — no confirmation, no guidance. Students were left wondering whether the feedback went anywhere and whether they still had to answer.

• exercise.html: a persistent note now appears under the "Is this English sentence confusing or unclear?" link after a successful submission: "Thank you for your feedback. Please answer with your best-guess interpretation of the sentence." It stays visible while the student answers, and carries role="status" so screen readers announce it when it appears (message is conveyed in text, no color-only signaling).
• ltltoengrater.html: on success, the modal closes and the note is revealed. A failed submission now shows an alert asking the student to retry — previously it only wrote to the console, making failure indistinguishable from success.
• Version bumped to 2.1.3; CHANGELOG entry added.

Why

Student-reported UX gap: flagging a sentence as unclear gave no acknowledgment and no instruction to proceed, so the report felt like a dead end.

Notes for review

• The question flow is intentionally unchanged: students still answer, and responses on flagged questions are logged normally. Excluding or down-weighting flagged answers in the misconception model would be a separate change to the answer-logging path.
• The rater modal is only included from exercise.html; the JS guards on the ack element existing, so the modal remains safe to reuse elsewhere.