Better feedback when file is missing from data_dir

[blakerosenthal]

populationsim/example_test$ python run_populationsim.py -c configs2

  File "/usr/local/anaconda3/lib/python3.7/site-packages/activitysim-0.9.2-py3.7.egg/activitysim/core/config.py", line 267, in data_file_path
    return cascading_input_file_path(file_name, 'data_dir', mandatory)
  File "/usr/local/anaconda3/lib/python3.7/site-packages/activitysim-0.9.2-py3.7.egg/activitysim/core/config.py", line 260, in cascading_input_file_path
    (dir_list_injectable_name, file_path, dir_list))
RuntimeError: file_path data_dir: file 'None' not in ['data']

[bstabler]

#397