Include error codes in schema for errors

We reference the error schema and have examples, but we don't say specifically that "this 403 can only return M_FORBIDDEN". Might be useful to also tweak the rendering of the error responses to reflect this

The issue does not name files or tests. Locate the error schema and the rendering path for error responses, then determine how status codes map to permitted error codes; it is done when the schema explicitly records the mapping and rendered responses reflect it.

Written by the indexing model from the issue text.