Implement missing HTTP status code pages

[Syndesi]

The folder src/Controller/Error/assets contains text files for all problematic status codes returned by the API, however at least one status code is missing: 405.

Acceptance criteria:

• Check whether there are other missing status code files.
• Implement missing status code files.
• Add missing documentation pages.
• Add tests to make sure these problem status code files can not get removed by accident in the future.