Add protocol error processing mechanism

protocol-spec/core.md

@@ -26,3 +26,42 @@ to (EntityID) - message recipient
 ok (boolean) - operation success indicator (used to determine if errors was happened when processing request)
 payload (Map<K,V>) - message payload (used to store extra information in message, list of permissible fields in the payload are depends on "type" field)
 ```
+
+## Errors
+Mechanism of error processing included into protocol.
+Adds into type name `:error` postfix.
+
+**Payload**:
+* `errCode: int` - error code (defined in extensions. Error code which defined in the specific extension MUST NOT overlap with other error codes)
+* `errText: String` - explanation of error in human-readable view
+* `errPayload: Map<K,V>` - advanced error information (fields defined in extensions)
+
+**Example**:  
+
+*Request*:
+```json
+{
+    "id": "abcd",
+    "type": "incorrectMessageType",
+    "from": "@juliet@cadmium.im",
+    "to": "cadmium.im",
+    "payload": {
+        "test": "test"
+    }
+}
+```
+
+*Response*:
+```json
+{
+    "id": "abcd",
+    "type": "incorrectMessageType:error",
+    "from": "cadmium.im",
+    "to": "@juliet@cadmium.im",
+    "payload": {
+        "errCode": 0,
+        "errText": "Incorrect type of message (type isn't implemented in the server)",
+        "errPayload": {}
+    }
+}
+```