feat: nullable exception handlers

[maximvlah]

Closes https://github.com/reflex-dev/reflex/issues/3776

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #5703 will not alter performance

_{Comparing maximvlah:set-none-to-exception-handler (7ceac10) with main (9b2d93e)}

Summary

✅ 8 untouched benchmarks

[maximvlah]

@masenf @benedikt-bartscher ready for review

[adhami3310]

the only issue is that this doesn't disable the frontend onError from triggering, it just that when it goes to the backend, it does nothing

[adhami3310]

i think to truly resolve the associated ticket, we do want to actually disable onError