feature: Allow any script to be used as flow error handler

[Bilge]

We are limited to a very specific subset of languages and scripts as error handlers but I expected we could use any existing script here. I expect the limitation is just because error handlers have a specific interface (message, name, step ID), but it seems to me this is not necessary at all, because we could route that data ourselves (if we actually want it) using inputs and interpolation as with a normal script.

Instead, we have to build duplicate scripts just to have them act as an error handler, which is a pretty poor user experience and a maintenance burden.