fix(next): removes reliance on instanceof from api error formatting

[jacobsfletch]

Description

Removes reliance on instanceof when formatting API errors from the REST API. This led to field-level validation errors to not get properly formatted in API responses, i.e. unique fields do not receive their individual validation messages. The reason was because previously, we were checking the error's instanceof in order to properly format the response based on APIError, ValidationError, etc. But changes to TS have made this condition unreliable (see https://dannyguo.medium.com/how-to-fix-instanceof-not-working-for-custom-errors-in-typescript-1df978100a27). Unfortunately the fix described in that article also does not resolve this issue, i.e. setting the prototype in the class' constructor. So instead, we check the name of the constructor explicitly. There might be a better fix for this in the future.

This PR also fixing the casing on the routeError function.

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