## Description
Currently, error type equality and error creation on builtin functions is quite primitive:
For Z850:
* Z850K2 only allows references, as it's lazily evaluated to be able to still get the error type zid and not the dereferenced error type object. Error type comparison cannot be done if the object is dereferenced because it lacks an identity key, so Z850K2 cannot be defined by a other than a Z9.
* We should build some heuristics to compare two error types similarly to how we do it with Z4s, for which we need Z50s to have an identity key.
For Z851:
* Errors are created given an error type and a set of string arguments.
* The error type definition is not checked to make sure that the arguments match the types of the error keys.
* Only string arguments are allowed, any other argument type is ignored.
* Error instances produced can hence be wrong (e.g. Z500 can have two keys, when it only defines one), so the responsibility lies in the hands of the editors.
For a sturdier error system:
* [ ] Add identity key to Z50 objects
* [ ] Write error type comparison logic that allows comparing between error types at different level of dereferentiation (recursively down their ID key till something comparable is found)
* [ ] Write error type-aware logic for building error instances
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## Completion checklist
* [ ] Before closing this task, review one by one the checklist available here: https://www.mediawiki.org/wiki/Abstract_Wikipedia_team/Definition_of_Done#Back-end_Task/Bug_completion_checklist