Improve Error description

This improves the description of the Error model by indicating that
error messages must be unique and cannot have contextual information.

api.json

@@ -1722,7 +1722,7 @@
         }
       },
      "Error": {
-       "description":"Instead of utilizing HTTP status codes to describe node errors (which often do not have a good analog), rich errors are returned using this object.",
+       "description":"Instead of utilizing HTTP status codes to describe node errors (which often do not have a good analog), rich errors are returned using this object. Both the code and message fields can be individually used to correctly identify an error. Implementations MUST use unique values for both fields.",
        "type":"object",
        "required": [
          "code",
@@ -1738,7 +1738,7 @@
            "example": 12
           },
          "message": {
-           "description":"Message is a network-specific error message.",
+           "description":"Message is a network-specific error message. The message MUST NOT change for a given code. In particular, this means that any contextual information should be included in the details field.",
            "type":"string",
            "example":"Invalid account format"
           },

models/Error.yaml

@@ -15,6 +15,10 @@
 description: |
   Instead of utilizing HTTP status codes to describe node errors (which often
   do not have a good analog), rich errors are returned using this object.
+
+  Both the code and message fields can be individually used to correctly
+  identify an error. Implementations MUST use unique values for both
+  fields.
 type: object
 required:
   - code
@@ -32,6 +36,10 @@ properties:
   message:
     description: |
       Message is a network-specific error message.
+
+      The message MUST NOT change for a given code. In particular, this
+      means that any contextual information should be included in
+      the details field.
     type: string
     example: "Invalid account format"
   retriable: