/invitation-accepted remove 404 status

In #58 this status code was added, I think it ought to be removed in favor of
the 400 status code that allready exists, so I made the message clearer.

spec.yaml

@@ -225,17 +225,13 @@ paths:
           schema:
             $ref: "#/definitions/AcceptedInviteResponse"
         400:
-          description: The invitation token is invalid.
+          description: The invitation token is invalid or does not exist.
           schema:
             $ref: "#/definitions/Error"
         403:
           description: Remote service is not trusted to accept invitations.
           schema:
             $ref: "#/definitions/Error"
-        404:
-          description: The invitation token does not exist.
-          schema:
-            $ref: "#/definitions/Error"
         409:
           description: User is already known by the OCM provider.
           schema: