err to failed

Communication.md

@@ -35,8 +35,8 @@ Commands from _execution thread_ to _listening thread_:
 
 - **done** - notifying of finished job. Requires 2 message frames:
     - `job_id` - identifier of finished job
-    - `result` - response result, one of "OK" and "ERR"
+    - `result` - response result, one of "OK" and "FAILED"
-    - `message` - non-empty error description if result was "ERR"
+    - `message` - non-empty error description if result was "FAILED"
 
 
 ### Progress callback