Document, homogenize, and improve error responses and JSON Schema Validation messages

[aeneasr]

Is your feature request related to a problem? Please describe.

We are currently lacking a consistent error strategy and need to homogenize payloads, document error types and their contexts, and so on. This makes several things hard:

• Identifying validation error messages (currently we use the HTTP status code...)
• Translating validation error messages. This would probably require a context object to e.g. identify a number: Your username must be longer than X characters

Additional context

Add any other context or screenshots about the feature request here.

[aeneasr]

It would probably be good if we have an iota for all errors, making breaking changes unlikely.

[aeneasr]

Make error IDs an array so that breaking changes are not as bad because one can use the legacy error code. This might happen when more specificity is added.

[aeneasr]

Resolved - more or less - on master. Still needs improvement but this can be done incrementally for each request. I think as soon as people complain about this we can point them to the right files to fix it.