Update agent-http-api.md

Author: zeegomo

docs/agent-http-api.md

@@ -60,6 +60,7 @@ behave this way:
 * `POST /agent-api/complete-experiment` should be called as soon as the agent
   has nothing left to do with the current experiment; after the method returns
   `next-experiment` will return a new experiment
+ * `POST /error` should be called only when the agent has encountered an error
 
 ## Available endpoints
 
@@ -227,3 +228,26 @@ The endpoint replies with `true`.
     "result": true
 }
 ```
+
+### `POST /error`
+
+This endpoint tells the Crater server the agent has encountered an error. The endpoint expects the error description to be provided as the request body, encoded in JSON:
+
+* `error`: a description of the error
+
+For example, this is a valid request data:
+
+```json
+{
+    "error": "pc is not powered on"
+}
+```
+
+The endpoint replies with `true`.
+
+```json
+{
+    "status": "success",
+    "result": true
+}
+```