servers should use 404

Author: Eh2406

src/doc/src/reference/registries.md

@@ -320,9 +320,8 @@ visit the registry's website to obtain a token, and Cargo can store the token
 using the [`cargo login`] command, or by passing the token on the
 command-line.
 
-Responses use a 200 response code for both success and errors. Cargo looks at
-the JSON response to determine if there was success or failure. Failure
-responses have a JSON object with the following structure:
+Responses should use the 200 or 404 response code as appropriate. Failure
+responses should have a JSON object with the following structure:
 
 ```javascript
 {
@@ -336,10 +335,11 @@ responses have a JSON object with the following structure:
 }
 ```
 
-Servers may also respond with a 404 response code to indicate the requested
-resource is not found (for example, an unknown crate name). However, using a
-200 response with an `errors` object allows a registry to provide a more
-detailed error message if desired.
+If the response has this structure Cargo will display the detailed message to the user, even if the response code is 200.
+If the response code indicates an error and the content does not have this structure, Cargo will display to the user a
+ message intended to help debugging the server error. A server returning an `errors` object allows a registry to provide a more
+detailed or user centric error message. The first release of Cargo to support registries Cargo 1.34 (released 2019-04-11) will only 
+show a human readable error if the code is 200, this has been fixed in all versions since Cargo 1.35 (released 2019-05-23).
 
 For backwards compatibility, servers should ignore any unexpected query
 parameters or JSON fields. If a JSON field is missing, it should be assumed to