cancelHoldInvoice failures are swallowed silently — user gets no feedback when LND is down

@ToRyVand is already working on this.

Since Aug 5, 2026.

## Summary

When `cancelHoldInvoice` fails (e.g. LND is temporarily unreachable), all call
sites in `bot/commands.ts` and `bot/start.ts` catch the error via their outer
`try/catch`, log it, and return — without sending any message to the user.
The operation simply doesn't complete, and the user has no way of knowing
something went wrong.

## Background

Prior to #884, `cancelHoldInvoice` swallowed LND errors internally — it would
catch the failure, log it, and return `undefined` as if nothing had happened.
Callers had no way to detect a failure, so they would proceed to mark orders
as `CANCELED`, `CANCELED_BY_ADMIN`, or `SELLER_REFUNDED` even when the hold
invoice was never actually canceled. #884 fixed this by making
`cancelHoldInvoice` re-throw LND errors, consistent with `settleHoldInvoice`.
This made the failure visible to callers — but exposed that the command-based
call sites don't handle it with user feedback.

## Affected call sites

- `bot/commands.ts:322` — seller cancels in `WAITING_PAYMENT`
- `bot/commands.ts:534` — buyer cancels in `WAITING_BUYER_INVOICE`
- `bot/commands.ts:711` — cooperative cancel
- `bot/commands.ts:790` — admin cancel
- `bot/start.ts:445` — admin cancel (alternate path)
- `bot/start.ts:534` — cancel all orders

## Current behavior

LND fails → error is caught and logged → user sees nothing → order stays
in its current state.

## Expected behavior

LND fails → user receives an error message explaining the operation failed
and they can retry.

## Notes

- The job-based call sites (`cancel_orders.ts`, `check_hold_invoice_expired.ts`)
are fine — they retry automatically on the next run.
- A generic LND error message function may already exist in `bot/messages.ts`
— worth checking before adding new ones.