More helpful message if --repo_path doesn't exist

Closes #763
Also see #761

sweagent/environment/utils.py

@@ -642,6 +642,13 @@ def get_instances(
     Returns:
         List of instances as dictionaries
     """
+    if file_path and not Path(file_path).exists():
+        msg = f"Specified problem statement path {file_path} does not exist"
+        raise FileNotFoundError(msg)
+
+    if repo_path and not Path(repo_path).exists():
+        msg = f"Specified repository path {repo_path} does not exist"
+        raise FileNotFoundError(msg)
 
     def instance_from_dict(instances):
         ib = InstanceBuilder(token=token)