Custom error classes with `error_if`

[f0uriest]

Right now error_if always raises an EquinoxRuntimeError. Is it possible to have this be user controlled, so it could be eg ValueError or some custom user error type? I saw #762 the warning category can be specified, but I know the runtime error stuff is tricky so not sure if there's a hard reason error_if couldn't be modified to allow this.

[patrick-kidger]

So the raise actually happens in filter_jit here:

equinox/equinox/_jit.py

Line 286 in 48453cc

raise EquinoxRuntimeError(

I think something like this is technically feasible, but I think I'd prefer to leave it out-of-scope unless there's a really compelling reason? It'd just be quite finickity to review/maintain.