Unsuccessful response format of `/keys/signatures/upload` is poorly specified

Link to problem area:

https://spec.matrix.org/v1.7/client-server-api/#post_matrixclientv3keyssignaturesupload

Issue

The response format for this is specced as failures | {string: {string: Error}} but doesn't really say what shape an Error is. The example implies it should be a standard error response, but neither the spec wording nor the openapi spec actually say so.

In practice, synapse returns a structure like this:

{
    "status": 400,
    "errcode": "M_INVALID_SIGNATURE",
    "message": "400: Invalid signature"
}

... which, other than an errcode, is nothing like a standard error response.

MSC1756, which introduced this endpoint, says simply "if a signature is not valid, the homeserver should set the corresponding entry in failures to a JSON object with the errcode property set to M_INVALID_SIGNATURE."

Start with the linked Matrix Client-Server API section for POST /keys/signatures/upload, then compare its response wording with cross_signing.yaml and MSC1756. Document the shape of each failures entry consistently across the prose, example, and OpenAPI specification, including what the Error object contains.