Unify response codes documentation

[arkaitzgarro]

The documentation doesn't explain the expected response codes in a nice way for some endpoints (like /authorize) and is even missing for some of them. We should have an unified way of documenting response codes and response body.

I would suggest to use the following form, as we do for some responses:

Responses

200 (OK)

The Response Body contains the pre-signed S3 upload url.

{
  "url": "https://presigned-s3-put-url"
}

404 (Not found)

If you try to request an upload URL for a file that is not in the transfers, the API will respond with 404 Not found.

{
  "success" : false,
  "message" : "Couldn't find FileObject"
}

417 (Expectation Failed)

If you request to upload part 0, our API will tell you that our parts are numbered for humans; we start counting at part 1:

{
  "success" : false,
  "message" : "Chunk numbers are 1-based"
}

[arnoFleming]

Also provide a link to our documentation in 4xx and 5xx all responses