Better error message.

run_coverage.py

@@ -132,7 +132,7 @@ def process_interface_version(interface_version):
     for primitive_id, primitive in known_primitives.items():
         if primitive_id not in primitives_used_in_pipelines:
             has_errored = True
-            print(primitive['source']['name'], primitive['id'], primitive['python_path'], flush=True)
+            print("Primitive is not used in any pipeline:", primitive['source']['name'], primitive['id'], primitive['python_path'], flush=True)
 
     return has_errored