Fixes#1820.
Correct error handling in response to CORS.
Changes proposed in this pull request:
- Add a MiddlewarePosition before Exception handling so CORS is always
returned
- Add ServerError Middleware to handle unhandled errors between the
ServerError- and ExceptionMiddleware
- Update corresponding docs
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Co-authored-by: Robbe Sneyders <robbe.sneyders@ml6.eu>