Add note about streaming error responses

Author: mjankowski

content/en/methods/streaming.md

@@ -687,6 +687,16 @@ An example filter change by the user:
 Note that the `payload` property is not present for `filters_changed` events. And for `delete` and `announcements.delete` the payload is a string, not an object.
 {{</hint>}}
 
+## Error responses
+
+The streaming server will respond to error conditions with an HTTP 4XX response and a JSON body describing the error. For example:
+
+```json
+{
+  "error": "Unknown channel requested"
+}
+```
+
 ## See also
 
 ### Streaming server