Add distinction between two unallowed cases in name check "eval_" (#489)

lerobot/common/robot_devices/control_utils.py

@@ -324,7 +324,15 @@ def sanity_check_dataset_name(repo_id, policy):
     _, dataset_name = repo_id.split("/")
     # either repo_id doesnt start with "eval_" and there is no policy
     # or repo_id starts with "eval_" and there is a policy
-    if dataset_name.startswith("eval_") == (policy is None):
+
+    # Check if dataset_name starts with "eval_" but policy is missing
+    if dataset_name.startswith("eval_") and policy is None:
+        raise ValueError(
+            f"Your dataset name begins with 'eval_' ({dataset_name}), but no policy is provided."
+        )
+
+    # Check if dataset_name does not start with "eval_" but policy is provided
+    if not dataset_name.startswith("eval_") and policy is not None:
         raise ValueError(
-            f"Your dataset name begins by 'eval_' ({dataset_name}) but no policy is provided ({policy})."
+            f"Your dataset name does not begin with 'eval_' ({dataset_name}), but a policy is provided ({policy})."
         )