If the user code raises a 400, don't try to override it.

I have a number of validators implemented as Argument `type`s to handle parameters values like `path?somelist=a,b,c` and `path?range=start:end`. if the validator fails, it raises a 400 with a contextual (rather than static, using `help`) error message. `handle_validation_error()` swallows these 400s, and this fix propagates them instead.