add abort message

IntroductionClient.md

@@ -19,7 +19,7 @@ The introduction protocol uses three message types. It should be noted that all
 
 **2: RESPONSE** - This should be send by contacts that received an introduction before based on user input. If the facilitator receives this, she should forward it to the other introducee with the correct introduction ID. The content is a BDF list with these elements:
 * (int) The type of the message, here **2**
-* (raw) The introduction ID which is the `MessageId` of the introduction message.
+* (raw) The session ID
 * (bool) Whether the introduction is accepted or not. The following list items are **only** included if the introduction was accepted
   * (int) The timestamp of the current time
   * (raw) An ephemeral public key which will be used to derive a shared secret (see [BPQ](BQP))
@@ -32,7 +32,11 @@ The introduction protocol uses three message types. It should be noted that all
 
 **3: ACK** - This should be send by a contact that previously responded to an invitation and has received the response of the other contact. If the facilitator receives this, she should forward it to the other introducee with the correct introduction ID. The content is a BDF list with two elements:
 * (int) The type of the message, here **3**
-* (raw) The introduction ID which is the `MessageId` of the introduction message.
+* (raw) The session ID
+
+**4: ABORT** - This should be send by a contact that encountered an error, so that it can not complete the protocol. The content is a BDF list with two elements:
+* (int) The type of the message, here **3**
+* (raw) The session ID
 
 ![IntroductionClient](/uploads/d3ba752c95f938395e7cf4566d107aff/IntroductionClient.png)