Implement RFC 7807 error handling with compatibility mode (#165)

[avoinea]

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[mister-roboto]

@avoinea thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[read-the-docs-community]

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[davisagli]

Oh, hmm. I hadn't even noticed Timo's suggestion in #165 to follow RFC 7807 -- and that was 7 years ago. I like the idea of following a standard, but I need to study the details before I give more feedback.

Initial reactions:

1. RFC 7807 says at the top that it is obsoleted by RFC 9457, so we should take a look at that one instead.
2. Changes to the serialization of exceptions should be done in https://github.com/plone/plone.rest/blob/main/src/plone/rest/errors.py, not in the Service class here.

[avoinea]

@silviupanaite Please see @davisagli suggestions.