Improving MetaCurriculum initialization.

- Raises MetaCurriculumError when curriculum_folder is not a folder.
- Removed the ability to set curriculum_folder to None.
  trainer_controller.py has been refactored to not depend on this
  functionality which will make curriculums more stable.

python/tests/test_meta_curriculum.py

 from unittest.mock import patch, call, Mock
 
 from unitytrainers.meta_curriculum import MetaCurriculum
+from unitytrainers.exception import MetaCurriculumError
 
 
 class MetaCurriculumTest(MetaCurriculum):
              call('test/Brain2.json', default_reset_parameters)]
 
     mock_curriculum_init.assert_has_calls(calls)
+
+
+@patch('os.listdir', side_effect=NotADirectoryError())
+def test_init_meta_curriculum_bad_curriculum_folder_raises_error(listdir):
+    with pytest.raises(MetaCurriculumError):
+        MetaCurriculum('test/', default_reset_parameters)
 
 
 @patch('unitytrainers.Curriculum')

python/unitytrainers/meta_curriculum.py

             default_reset_parameters (dict): The default reset parameters
                 of the environment.
         """
-        if curriculum_folder is None:
-            self._brains_to_curriculums = None
-        else:
-            used_reset_parameters = set()
-            self._brains_to_curriculums = {}
+        used_reset_parameters = set()
+        self._brains_to_curriculums = {}
+
+        try:
+
+                # Check if any two curriculums use the same reset params.
-                    logger.info('Two of more curriculums have attempted to change '
-                                'the same reset parameter. The result will be '
+                    logger.info('WARNING: Two of more curriculums will '
+                                'attempt to change the same reset '
+                                'parameter. The result will be '
+
+        except NotADirectoryError:
+            raise MetaCurriculumError(curriculum_folder + ' is not a '
+                                      'directory. Refer to the ML-Agents '
+                                      'curriculum learning docs.')
 
 
     @property

python/unitytrainers/trainer_controller.py

                                               'defined in ' +
                                               self.curriculum_folder + ' '
                                               'does not have a corresponding '
-                                              'Brain. Please check that the '
+                                              'Brain. Check that the '
                                               'curriculum file has the same '
                                               'name as the Brain '
                                               'whose curriculum it defines.')