docs(errors): document the different behaviour depending on status code (#1359)

Author: odoucet

core/errors.md

@@ -101,3 +101,10 @@ the error will be returned in this format as well:
   "hydra:description": "The product \"1234\" does not exist."
 }
 ```
+
+## Message Scope
+
+Depending on the status code you use, the message may be replaced with a generic one in production to avoid leaking unwanted information.
+If your status code is >= 500 and < 600, the exception message will only be displayed in debug mode (dev and test). In production, a generic message matching the status code provided will be shown instead. If you are using an unofficial HTTP code, a general message will be displayed.
+
+In any other cases, your exception message will be sent to end users.