chore: do not throw when cannot propose

[just-mitch]

See comment in L1Publisher

  public async canProposeAtNextEthBlock(archive: Buffer): Promise<[bigint, bigint]> {
    // FIXME: This should not throw if unable to propose but return a falsey value, so
    // we can differentiate between errors when hitting the L1 rollup contract (eg RPC error)
    // which may require a retry, vs actually not being the turn for proposing.