Add exception for in-edtior training

python/learn.py

 from docopt import docopt
 
 from unitytrainers.trainer_controller import TrainerController
-
+from unitytrainers.exception import TrainerError
 
 if __name__ == '__main__':
     print('''
       --lesson=<n>               Start learning from this lesson [default: 0].
       --load                     Whether to load the model or randomly initialize [default: False].
       --run-id=<path>            The sub-directory name for model and summary statistics [default: ppo].
-      --num-runs=<n>             Number of runs of session [default: 1]. 
+      --num-runs=<n>             Number of concurrent training sessions [default: 1]. 
       --save-freq=<n>            Frequency at which to save model [default: 50000].
       --seed=<n>                 Random seed used for training [default: -1].
       --slow                     Whether to run the game at training speed [default: False].
     base_path = os.path.dirname(__file__)
     TRAINER_CONFIG_PATH = os.path.abspath(os.path.join(base_path, "trainer_config.yaml"))
 
+
-        tc = TrainerController(env_path, run_id+"-"+str(sub_id), save_freq, curriculum_file, fast_simulation,
-                               load_model, train_model, worker_id+sub_id, keep_checkpoints, lesson, seed,
+        tc = TrainerController(env_path, run_id + "-" + str(sub_id), save_freq, curriculum_file, fast_simulation,
+                               load_model, train_model, worker_id + sub_id, keep_checkpoints, lesson, seed,
+
+    if env_path is None and num_runs > 1:
+        raise TrainerError("It is not possible to launch more than one concurrent training session "
+                           "when training from the editor")
     jobs = []
     for i in range(num_runs):
         p = multiprocessing.Process(target=run_training, args=(i,))