use absolute path in error (#3230)

ml-agents/mlagents/trainers/trainer_util.py

+import os
 import yaml
 from typing import Any, Dict, TextIO
 import logging
         with open(config_path) as data_file:
             return _load_config(data_file)
     except IOError:
-        raise TrainerConfigError(f"Config file could not be found at {config_path}.")
+        abs_path = os.path.abspath(config_path)
+        raise TrainerConfigError(f"Config file could not be found at {abs_path}.")
     except UnicodeDecodeError:
         raise TrainerConfigError(
             f"There was an error decoding Config file from {config_path}. "