Function validate_profile 

fn validate_profile(
    outstanding: &[f64],
    periods: usize,
) -> Result<(), CurveError>

Checks a notional profile: exactly one fraction per coupon period, the first 1.0, every entry finite, positive and no greater than the one before it. Shared by every bond that carries one, so the refusals read the same whichever product they come from.