Error when converting non-finite values to a finite-only type

[AntonOresten]

Basically maybe we shouldn't error in any case, since hardware-accelerated implementations will never do this:

iirc, PTX .satfinite semantics means converting NaN to positive floatmax.

[AntonOresten]

I'd argue that this is fine for a generic fallback. Deviations from hardware instructions might be fine since they might be vendor-specific anyway, and we can't afford too many dials. Open to returning positive floatmax though.

[AntonOresten]

Comes up again in #31