Return RFC7807 type errors

[sagikazarmark]

Preflight Checklist

• [X] I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

Currently we return a custom error format to clients. While that's not necessarily a problem, some standardisation would be nice.

Proposed Solution

Return errors formatted according to RFC7807.

A library I've used in the past: https://github.com/moogar0880/problems

Alternatives Considered

Given the size of the library, we could just implement our own problem types.

Additional Information

No response

[sagikazarmark]

@tothandras is this done?

[tothandras]

@sagikazarmark let me know if you find this acceptable:

• https://github.com/openmeterio/openmeter/blob/next/api/openapi.yaml#L173
• https://github.com/openmeterio/openmeter/blob/next/pkg/models/problem.go

[sagikazarmark]

lgtm