`errorHandler` is unusable

[SirNickolas]

import argparse;

struct Args {
    bool banana;
}

mixin CLI!({
    Config cfg;
    cfg.errorHandler = (string) { };
    return cfg;
}(), Args).main!((in _) { });

argparse/config.d(90,9): Error: closures are not yet supported in CTFE

N.B. (string) { } is not a closure; return errorHandlerFunc = (string msg) { func(msg); }; (inside the library) is.